I noticed in one of my group chats that my SMS email gateway was one of the participants.
That made me think that the way I get my own texts sent back to me is through my SMS email gateway.
So I sent myself a text to my SMS email gateway (which would be yournumber@vtext.com).
I then blocked that recipient, i.e. my own sms email gateway, And I stopped receiving my own texts back in Group Chats.
This worked for me in Google Messages.
Unfortunately, I can’t figure out a way to Block a number in Message Plus.