Client confused about the membership of a room

[richvdh]

We've seen a number of reports of UTD events where the sender was using Element iOS, and the sending client has gotten confused about the membership list in the room (thinking that there are many fewer members than there actually are).

In one case, the client seems to have spontaneously forgotten the majority of the room membership:

2025-01-29 08:06:33.977 Element[13714:2050845] 13:06:33.977 [MXFileStore commit] saveRoomsState: saving 33 events for room !<redacted>:matrix.org
...
2025-01-29 08:06:43.848 Element[13714:2046936] 13:06:43.848 [MXFileStore commit] saveRoomsState: saving 13 events for room !<redacted>:matrix.org

Initially I thought this might be related to matrix-org/matrix-spec#1209, element-hq/synapse#17050 or element-hq/synapse#16940, but the symptoms don't seem to fit: the period above does not correspond to a particularly complex bit of event graph.