Why are my MMS messages "downloading" or showing in the wrong threads?

[AC Question]

I have a t-Mobile Galaxy S6. I upgraded to Marshmallow in mid-May. Ever since then, any MMS messages (both groups and picture texts) are showing up in the wrong threads (i send to 3 specific people who reply back, so I know they received the text. But the message is only visible in a thread between completely different people.) Or I get a "Downloading" message that never comes through.

i am using only the messaging apps that came on the device and I had no problem until the day I upgraded to Marshmallow. I have tried deleting the threads and deleting the cache.

Please HELP!!! I use my phone for work and group texts are a necessity...