Sliding sync doesn't seem to correctly initialize new rooms in some cases

Observed in some rageshake: Cannot post process a room in sliding sync because it is missing, which corresponds to this function. This is weird, because all the joined room responses should've lead to the creation of a room in the store, so it looks like we're losing track of some rooms, somehow.

The corresponding line shows that the room only got read receipt updates, so I suspect that if a room is mentioned in the receipts subsection of the response, but not in the rooms section, then it's not correctly set up (the Room and RoomInfo are not getting created). I wonder why it wouldn't be in the rooms section in the first place; maybe it's outside the range of the sliding sync room list? Something to investigate a bit more.