I'm having issues with the stock Samsung messages app in my T-Mobile S9. It seems they rolled out some RCS-type upgrade and now when I create a group MMS, some responses come in normally to the MMS and other come to a Group chat created with the same people?!? So I end up with some responses in one group, and some in another group that is recognized as a chat (even puts a nice icon with a "head" next to the messages). Any ideas? If I switch to Android Messages or look at the texts in the Digits web interface it's all fine... it looks like Samsung messages is processing some replies as MMS and others as RCS-chat into two different groups. Any ideas?
These are features of your Carrier. MMS configuration and settings are modified by carrier so I think it's better you contact TMobile or just use 3rd party message app. You can also clear the data of the app to put back the settings to default.