iPhone Group Texts Issue

I have an iPhone 6; the phone is a bit less than two years old.

A few days ago, I confirmed that I had temporarily stopped receiving texts in groups that involve both iPhones and non-iPhones. I can still text back and forth with non-iPhones individually, and I can send and receive texts in groups containing exclusively iPhones. It's the mixed groups that present the issue.

I did some internet research two days ago, and I did the following: (1) updated phone software; (2) reset network settings; (3) deleted all existing texts; and (4) did a hard reset (several times).

Then, in the middle of the night, I received 100+ texts that had been sent in the problematic groups, and the issue appeared to be resolved. However, today I confirmed that the issue has returned, and I am again not receiving texts from the mixed threads.

I used the TD search function but could not find a previous thread on here with this topic / discussion. Any suggestions or information are appreciated, as this is a very frustrating problem. I contacted AT&T customer service - utterly worthless as anticipated.

THANKS!