Check rear derailleur hanger alignment.

The alignment is right. It worked perfect before I swapped the rear to my Tt bike and put this one on. Like I said, it shifts into the 27 fine but then shifts down.

Low limit screw too tight

Sorry to be pedantic, but unless you verify alignment with an alignment tool, you can't be sure. The act of changing rd's could have moved the dropout.

I'll check the alignment and the low limit.

I realized that I wasn't perfectly clear. When it shifts 10 seconds later, it's an actual shift. Not just the chain jumping off, but a perfect shift to the next smaller cog.

Do you have an 11 speed cassette body? If so, you need the 10 speed spacer.

Also, on the 11 speed rear der, it will hyper shift the last cog, then move down a little after 10 seconds or so. It does the same thing with the smallest cog, but moves up. It's to make sure that it gets in to the cog, and adjusts so that it is better alignment. I believe it does this because there is often so much load on the chain in these gears.

10 speed body. It doesn't have anything to do with the wheel or cassette because I even put a wheel on with no cassette and went through the shifting. It still does the downshift.

I would suggest, as others said too, to make sure the rear der hanger isn't bent. A shop would have to check that.

It's a new hanger. And it doesn't seem to be an alignment issue. Like I said, it does the shift even without a wheel. It seems to be an electronic issue more than an old fashioned adjustment issue.

Every new hanger I've ever installed has needed an alignment. The interface between the frame and the hanger is never perfect. I urge you to get an alignment tool (for example the park dag-2) and check this. Never assume it's good. Di2 is particularly sensitive to alignment issues. Why is there a new hanger?

Good question. I changed it because I was having trouble getting the shifting perfect and that did help. No crash or anything. I'll take it and have someone look at it.

Can you shift into the large cog then pull the shifter button plug(s)? - Just to rule out switch debounce issue.

Also shift into the large cog then pull the battery - to rule out electronic vs something mechanical

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I pulled the plug after shifting and then it stays in the largest cog. Actually it didn't even change when I plugged it back in. But then after shifting down and back it does it. That's how I know it's not an alignment or adjustment thing.

Pulled what plug?

A) Main trunk cable to headset?
b) Shifter button(s), RD only?
c) battery from holder?

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Training Tweets: https://twitter.com/Jagersport_com
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Have you tried winding out the limit screw? Maybe the the RD is feeling a bit of resistance when it shifts into the largest cog against the limit screw and then 'decides' it's not right and down shifts. Maybe give the RD a thorough clean in case a bit of grit has got stuck somewhere.

There is nothing that functions like that on di2. It's either properly aligned or not. As others have said, this is either limit adjustment off or hanger alignment. Nothing you have done here has ruled either of these out. There are no "smarts" in the Rd that decide what to do.
If you wind the limit out by one turn does this change the behaviour?

With rear mech cable out-doesn't do it. Any other disconnect battery,shifter, junction box-still does it. I'll play with the limits. It's a powered shift. I can hear the motor when it changes.

I appreciate all the help.

Something I did fixed it. I played with the b limit and low and high limits. I'm thinking the upper jockey wheel was too close and it recognized that and shifted. Again, thank you for the thoughts.