I posted this a while ago. I found the answer in the following thread after puzzled over it for a couple days and spent an hour with an online tax expert.
Here's the thread where I finally figured out this was a bug:
https://ttlc.intuit.com/community/retirement/discussion/word-rollover-does-not-appear-next-to-form-1...
Here's the procedure that is correct.
https://thefinancebuff.com/mega-backdoor-roth-in-turbotax.html#:~:text=A%20mega%20backdoor%20Roth%20...
However, the word "ROLLOVER" may not appear if you are using the online Turbotax at least. I did print out all the worksheets and it does seem the proper boxes are checked for a Roth in-plan rollover on the Turbotax worksheets if I answer according to the above procedure. But it does not put "ROLLOVER" by box 5b.
Essentially I just decided to give up and leave it without the word "ROLLOVER". It took me quite a while to figure out what was going on until I found the first link above that confirmed it was a bug. Not very satisfying and makes me less confident in Turbotax (plus I wasted a lot of time), but at least I know I entered my data correctly even if Turbotax doesn't update the 1040 perfectly. I do think they need to consider improving their support for Mega Backdoor Roth in-plan rollovers, they are very common now at tech companies.
