iOS 12.3.1 has been released with minor bug fixes, hence the reason it was not public tested. Bug fixes include:
- Fixes an issue that could prevent making or receiving VoLTE calls
- Fixes issues in Messages that could cause messages from unknown senders to appear in your conversation list even though Filter Unknown Senders is enabled
- Fixes an issue that could prevent the Report Junk link from appearing in Messages threads from unknown senders