Matrix /sync deltas don't include profile-only changes
Building a Matrix sync agent/bot that reads room timelines incrementally and needs sender display names alongside events.
Matrix has two ways a user's display name can change: (a) a m.room.member event in a specific room (e.g., join, leave, change-name-in-this-room), which appears in /sync timeline and state deltas, or (b) a PUT to /profile/<mxid>/displayname, which updates the user's GLOBAL profile and emits a fanout of member events INTO every joined room... but only AT THAT MOMENT. If your /sync agent was running with since=<next_batch> BEFORE the profile PUT happened, you got the fanout member event and saw the name. If you joined a room (or started syncing) AFTER the PUT, you DON'T see the original profile change as a state event — Matrix only includes state events that fell within the sync window. Long-running agents that build their (mxid → display) map purely from /sync deltas will therefore see displays drift to null over time as bridge bots set names on puppets via profile PUTs that happened during gaps. The diagnostic is precise: /profile/<mxid> returns the correct name, but roomMembers map from the /sync response doesn't have it. Fix: after each /sync iteration, identify senders whose display is missing from the in-response state, fetch /profile/<mxid>/displayname for each (cached for the process lifetime), inject as a synthetic member event into the in-memory sync data so existing code paths pick it up. Cost: a few /profile calls per process lifetime, never per-event.
Don't treat the Matrix /sync state map as authoritative for display names — it's a snapshot conditioned on what fell in the delta window, not the current truth. If you need authoritative current displays, fetch /profile lazily on miss and cache. Any Matrix sync agent should have this fallback from day one when displays matter (e.g., when feeding events into another system that records sender names).