This is a weird one and causing major problems for me. This occurs with both the stock messages app and with Quik from F-Droid. In MMS messages, a particular participant shows up with a "phone number" which is a concatenation of seven phone numbers. When I send messages to the group, they are not received by this contact.
When I look at the individuals' contact in the Contacts app, the number appears normal.
If I make a different MMS group including this individual, the number also appears normal; but if I try to re-make a group of the same composition, they appear again with the broken "phone number".
Before switching to GrapheneOS I had been using Google's RCS chats. Since switching, I used the Google Messages app to opt out of RCS. However, I believe this issue arose at the time I quit the Google Messages / RCS system.
However, I have validated that the issue exists within the OS's messages database, since exporting the messages using the "SMS Import / Export" app in F-Droid shows the erroneous "phone number" in the exported messages.
Has anyone else observed this behavior?
I may resort to deleting everything, fixing it in the export, and then re-importing. We'll see if it persists then.
On Pixel 9a in case it's relevant.