Recently put /e/OS on a fairphone 5. Everything works great besides trying to send individual texts to just one of my contacts in Message.
When I go to message just him it tries to send a text to an old group chat (nearly a decade old) but the messages aren’t received by anyone in the group. I can call this contact with no issues. Any group chat I add him to results in not sending to anyone but appears as the old group chat (all 5 numbers) as his contact. I deleted the contact and remade it with no luck.
I am using Tmobile in the US and have called support and verified the SMS and MMS settings are correct and APNs are configured as expected. The only other thing I can think of is something went awry with my text message backup when I got the phone. I used the SMS backup and restore app to do so from an old Samsung s8.
Does your contact use an iPhone or stock Android which is up-to-date? If so, there may be a factor of RCS involved as it caused me some problems with a group chat as well with e/OS on FP4. Somehow the stock Message app seems to work now, maybe due to me re-initiating the conversation after my iPhone buddies might have deleted the conversation on their devices so it was re-initiated. I am not sure but am happy to report that things “just work” right now.