I have the same problem with my 945.
I contacted Garmin who said they were working on it and then when dark on me.

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TEAM ZOOT 2020

Manually update calibration factor and save. I never had post-run calibration work for me.

https://support.garmin.com/...dN9X76LJ0N9MCG0CivpA

This appears to be for a footpod?
I'm using just the watch. Used to work great but now it sucks. It's not even reliably off. The more runs on the mill I do at the same pace I appearenly the faster I get. Running at about 830 and earlier I may have set a new 5k world record!!

My Garmin 945 is crazy off, also like 4min/mile when I'm running 8.

It hasn't gotten any better with running a lot of treadmill miles and choosing the 'recalibrate ditsance' option after the run to try and teach it the correct pace - it always goes back to its nuts pace and doesn't improve whatsoever.

My outdoor runs also haven't made a dent in the bizarro pacing, either.

It's not a main feature for me, but yes, it's def disappointing to see this watch feature literally be useless (or worse than useless) on such a premium product.

Might have fixed the issue. This is what worked for me so far:
Reset treadmill settings to factory. Not the whole watch, just the treadmill settings.
Did a couple outdoor runs with GPS. 1 was 3 mile and the other was 6.5 mile trail run.
Ran on treadmill. All is now good within 10-15" per mile. Good enough.
See how long it lasts.

Do you know off the top of your head how to reset the TM settings to factor? I need to do that big time.

I have a 935 but I would assume the same menus.
Select treadmill from your activity list.
Hold up button to access treadmill options
Select treadmill setting
Scroll down to restore defaults

Once you do that do some outdoor runs first

Thanks - I just did the "restore defaults" option. Will do some outdoor TM runs first and hope for the best!

I dunno about your 935 vs my 945, but do you still do the 'end run and calibrate' after each TM run and manually enter the distance, or do you just end run and use what it gives you and hope for the best? My manual seemed to suggest that if you manually enter the TM distance with 'end run and calibrate' that it should improve the accuracy, but so far that has utterly failed for me and I'll do what you're doing if it's only 10-20sec/mile off.

When I ran the first time after the reset it was close enough that I didn't need to calibrate again. It's important to have a similar arm swing when running indoor/outdoor for it to be accurate. I think the end run and calibrate is useless. The only thing it did was change my recorded distance on only the Garmin app. It would still export inaccurate info to both training peaks and strava which them I would have to go back and edit. Although as of late you can't edit strava activities anymore with free account so it is what it is i guess. Let me know how you make out.

Thanks for the tips.

Yes, I found the same issue with my Garmin 945 - I'd manually edit the TM distance after using the "End and calibrate" after the TM run, and all it did was change the final distance, leaving my world-record breaking 3 min/miles untouced on the record and not calibrating toward reality on subsequent runs.

That's funny, mine goes the other way, literally 50% slower (I run 1.5 miles before the watch clicks over 1)