Heart rate is only a surrogate for the power zone. It can be affected by hydration, fatigue, caffeine,...., so it isn't as helpful as just training by the power zones determined by your FTP test.

I monitor my HR versus power as an indicator of fatigue and there are days when there is zero chance of me holding my FTP heart rate but I can hold the appropriate power.

Agree with grumpier.mike’s comments.

One thing to also consider when training indoors and looking at heart rate is the temperature situation. A cooling fan can have a dramatic impact on heart rate and corresponding power. Though this obviously isn’t your issue as you believe your HR is reading too low. If I were you I’d do another FTP test as that will give you the answer you are looking for (has my power increased). No downside to testing!

Are you a good runner, or primarily a runner, or just moved to the bike from running?

When your heart rate does not respond to the power output (negative decoupling) it usually indicates that your cardio-vascular system is bigger than the muscles that turn the pedals (which are different than the ones that allow you to run.

When I run hard I can get the Garmin connect TE measurement up near 4, and over 4 in a race.

When on the trainer doing an FTP test it usually measures in the low 2's. My cycling legs can't put a tax on my heart and lungs - they just aren't strong enough,

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"...the street finds its own uses for things"

I swam competitively for 11 years (ten years ago) and played water polo in college. I'm guessing my legs just need to develop more. I have only been riding for about 5 years now (swimmer for 21 years)

Silly me, I discounted swimming :)

Yep, you have a huge cardiovascular capacity and no legs for biking. It doesn't mean the workouts are helping you improve, because they are, you are just working the leg muscles a lot more than the whole body. The trick is bring the legs up to the CV system, not the other way around. Keep swimming and running.

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"...the street finds its own uses for things"

First of all you do not have a problem.

What is your 60min power and weight? It looks like you have very well developed aerobic system vs anaerobic , which is good or even great. Most people have opposite.
This is perfect example how mathematical power training zones can't accommodate everyone...(not to start any comments)

It all depends what is your goal and what you are training for, if you want to increase power quickly, road racing, crits etc., 2 months of HIIT training would probably bring massive gains in your case, with big aerobic engine and some higher intensity training you would become monster.

Most of the roadies (time constrain athletes on time constrained plans) have massive anaerobic engines, but can't last longer during aerobic rides, burning sugar like gasoline. It is much harder and it takes way longer to develop solid aerobic engine, anaerobic training is fairly quick.
For starters add one high intensity session a week to your plan and see if it will change how you feel.

I recently addressed this problem with my coach as we were not seeing gains in my threshold power this year due to the "gap" in my zones as you illustrated. This problem became evident by comparing heart rate, perceived exertion, and power readings.

To find the new zones, I did two all-out efforts to determine the highest level of power I could withstand without dropping beyond a certain power cut-off point (you need to set this arbitrarily where you think your heart rate and RPE match power). In my case, it ended up being about 15 watts above my calculated threshold power from previous 15 mi TT assessments. We then adjusted my threshold power to match that point and have been working on HIIT above it for the last four weeks. At first, I had a couple of failed workouts to maintain the threshold power, but within two weeks I can consistently stay above it. Now, I see my heart rate matching my perceived exertion and power numbers. I am also benefiting by raising my real threshold power as well.

I am preparing to take December off, but we are going to reset the zones again with this method when I restart in January.

My FTP is the same as yours give or take but i have an even more exaggerated low HR, sub 120@200W, 130@240W, 150@300W. Collegiate running background. I've just always had a low HR. My max HR is only high 180s, so compared to someone who maxes at 200+ my zones seem to just be scaled down. Or I need to htfu and ride harder...

I found that when I dropped high intensity and went to IM training I lost overall bike fitness quickly despite large volume. I think there is a lot of truth in the poster above me about benefiting a ton from hard intervals and that's what I'm playing with now, tbd.

Also, my max HR is the same for running and biking so I don't think it is as simple as not having strong bike legs.

I somewhat have the same problem, when I'm doing my TR workouts, my H/R might get up to high 140s/ low 150s and I feel like I am dying!!! before when I had first started cycling my HR would see as much as mid to high 170s during a sprint, I might hit that when outdoors but seriously unable to ever go 160s (indoors) and hold as I normally do when riding outdoors!

during TR workouts I seem to tank around mid to high 140s honestly and I am not sure why! currently I am on the short power build (mid vol) also I have started to run early in the mornings and initially I was trying to run (very fast for my pace :-) but then decided to slow down & to build my run base, add mileage and this how it has been since... I don't know why!!!

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Speed kills unless you have speed skills!!!

I have never done a 60 min power test, I typically do the 20 min test. The last time I did the test my power was 304, which extrapolates out to a 60 min power of 289. Earlier in the year I had a 20 min power of 320 so I saw some decline throughout the summer for some reason. Trying to get that number higher for racing season by June (so I have a while). My weight is typically 165-170 lbs.

Next summer I plan on doing a local triathlon series. The races I have planned are sprint or olympic distance, so I basically want to build my power as high as possible.

Any recommendations for cycling workouts? I typically do 2x1 hour HIIT sessions a week with varying levels of time/effort levels, with a 1.5 hour lower intensity session on the weekend. I also have been doing 2-3 runs a week with a long run on the weekend (10-12 miles).

Did you stop making progress? If not, keep doing what you doing. It is impossible to suggest meaningful workout without knowing full picture (muscle type,lactate/vo2max/spirometry/primary cycling muscle group/current plan/training time any many other factors). Bottom line: for Tri you want to burn as much fat as possible during racing, so big aerobic base is a key, it takes time to built it. Adding high intensity at the right time takes your power even further up.

If you are only doing 2x1h HIIT and 1x1.5h low intensity cycling, total of 3.5h cycling you are benefiting from run or swim. Talk to the coach, they should be able to evaluate you much better, and see if your low HR is due to big stroke volume or something else.

Probably low stroke volume....resting HR on my last EKG was 37....