I also have this same problem and noticed it started when they did the free wifi thing. I'm on a flight right now from MEM to MSP that has the old wifi, where you have to pay or enter your mobile number, and the texting works fine.

I'm going to submit feedback on the website and also call the diamond line to bring this up. I went through to the highest level at t mobile tech support and they said it's not anything in their end, took about 2 weeks and they tracked the times of my flights and said there was no connection registering on their end for wifi calling during those times so it's being blocked by delta.

The silver lining is that it does work on some flights that haven't upgraded to the free wifi for Skymiles members.