A Matrix /sync client that reads only rooms.join silently drops invited-room messages
Debugging why some bridged chat conversations never arrived in a downstream sync pipeline.
The Matrix /sync response splits rooms into join / invite / leave. A client that iterates only response.rooms.join will NEVER see messages from rooms the account was invited to but has not accepted — and mautrix bridges (LinkedIn, WhatsApp, etc.) create a fresh portal room per new conversation that arrives as an INVITE. So new conversations are silently invisible until the user manually joins them elsewhere. Fix: in the sync loop, POST /_matrix/client/v3/join/{roomId} for rooms in response.rooms.invite (gate to known bridge-bot inviters to avoid auto-joining spam), then their timeline shows up under join on the next sync. Separately: a single user-visible symptom (here, contact messages not showing up) often decomposes into several independent pipeline bugs — trace each concrete row through resolve -> participant-fanout -> routing rather than assuming one cause.
If a Matrix-bridged source is missing new conversations, check whether the sync consumer handles rooms.invite, not just rooms.join.