Most of this story around the Timeline that has missing events is because gaps are loaded in the RoomEventCache but are not reflected in the Timeline. See #4632 for the whole story.

By the way, #4632 fixes the problem, but one subtle case can happen and recreate this behaviour:

• open a room (so a Timeline, so a RoomEventCache)
• the last chunk will be loaded by RoomEventCache
• the sync happens, and receives a limited response
• the RoomEventCache contains items, then a gap, then items
• this gap isn't resolved

Solution: when a limited response is received, the RoomEventCache should reload/shrink to a single chunk. This is also required by some apps, like Element X Android, that needs to shrinks the RoomEventCache due to memory pressure and Timeline performance on some devices. One seed, two birds.

• Address [meta] EventCache storage #3280