I have the same issue with my Galaxy S9 Plus since I got it a few months ago I get "failed" message when trying to send MMS messages both on wifi and using mobile data. I have to send photos a lot in my line of work and if this issue is not fixed I will have to switch to iphone. So frustrating!!!
I have same issue, the only way it replies is if I go to Device Care:Memory, then clear that memory and hit resend message and msg goes thru. After a couple of messages I have to go back and clear the memory because I start getting same message again. Its horrible but is what I have to do to reply to a group message with the new update.