I've seen this same issue with Google Messages. If I switch to Samsung Messages, or to Textra, the problem persists.
There's a thread reporting the same issue on a Google Messages forum with about 300 replies, all complaining about the same thing.
It's very odd that the issue is across different phones and text messaging apps, related to the same core messaging infrastructure.
I don't see the issue in Google Chat or Hangouts or Google Voice.
having same issue. After 2 hours with ATT they say its an android thing "happens all the time with all android devices"
I only had problem when using wifi calling. - ATT says that android wifi calling requires phone to switch to over the air to send MMS's (pics and group messages.
With that said I think I found the fix for me!!!!
I went to settings, apps, then top right corner and clicked app permission manager. Then tapped sms whiched showed 10 apps with permission. I allowed Amazon alexa to read and send texts so I could use my echos to send texts (never once actually used it) I turned off this permission and BAM! worked flawless past few days!! I would love to hear if this works for you