Mine would randomly check/uncheck.This very thing happened to me after updating to 15.4. Went back and forth between T-Mobile and Apple, each blaming the other, reset network, reset phone TWICE until that whole iCloud outage was resolved and it coincidentally started working again a few days after it broke. Now it's just started happening again to me. My phone has been on 15.4.1 since it rolled out but it only started this nonsense again today. I too have a T-Mobile e-sim only. May go by the store to pick up a physical sim tomorrow if it hasn't sorted itself out. This is definitely a widespread bug that neither party has really acknowledged and sought to resolve.
Has swapping to the physical sim continued to work for you?
I noticed this when I texted a new number and they asked me for my phone number. I was confused since I had just texted them and assumed they could just see it but sure enough, my email was being used instead of my phone number for iMessage. Same thing with FaceTime. I noticed this morning when I tried to make a call from my Mac using my iPhone. It gave me an error saying that I need to be using the same Apple ID/iCloud account except I was using the same Apple ID and iCloud account. Only issue is FaceTime won't activate my phone number for me.Interesting thread - I have a friend who is on T-Mobile and I have a group iMessage thread with him on it. For some reason, a few weeks ago, his email was somehow started being used instead of his phone number on our thread. He was not receiving any of our group messages. Changed it back to his phone number and all is well. I wonder if the issue is on T-Mobile side and not Apple?