This seems to happen to some users and not to others. Unfortunately on my side there’s nothing I can do.
When Join sends an SMS it simply uses the standard Android SMS sending API. Join can’t insert it directly into the phone’s SMS database because it’s not the default SMS app on your system. Only default SMS apps can write SMS messages to your phone’s SMS database.
So it’s the default app’s responsibility to write the SMS to the phone’s database.
If this happens to you in Textra one user that already contacted the Textra devs said you could go to Textra > Settings > Resync and that fixed the issue for him. Worth a try 🙂