We have actually seen some reports of this issue here in the community in the past. This issue is on the network side and we would need to gather some examples of this and open up an engineers trouble ticket to have it investigated. If you have some time, please contact us so our tech care folks can get the ball rolling on this for you.
I finally convinced my Fiancee to switch over from Verizon. She got the Apple XR. She can send group texts but responses are delayed anywhere from 30 minutes to 48 hours. We have spoken to T-Mobile tech support (ended up telling us to call Apple), Apple Support (told us it is a network issue), and replaced the SIM card. Resetting the network settings seems to help for a few minutes, but it ends up reverting back to the delayed responses within minutes. There is not a correlation between T-mobile only vs. other carriers and doesn't seem to be a correlation between ios and Android. T-Mobile tech support has been stumped and I'm trying to figure out how to move a couple levels higher in their organization to get someone who might have a little more creativity.