I am having the same problem with my Samsung s9+. I bought it unlocked and I use straight talk and I've never had a problem with them before and it's just the group messages. When I got my phone in November, I didn't have issues with the group chats and it started in March 2019
I also am having the same problem since the last update. I have been patiently waiting for the monthly patch to fix this, however the current Secuirty patch issued is April....come on folks its middle of June!