Invites are not always included in sync

[matrixbot]

This issue has been migrated from #7406.

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Description

On fast changing membership states the sync will not reliably include rooms a user was invited to.
It might be related to issues
matrix-org/synapse#4533 and
matrix-org/synapse#4422

Reproduction

This is not easily reproducable, an attempt should look as follows:

• Roomadmin a invites user b into rooms r_a and r_b
• a kicks b from r_a before he accepts the invite
• a changes room_state of r_b multiple times
• b retrieves current rooms via (initial)sync, r_b doesn't show

Version information

We used the synapse version v1.12.0.
Installed per git clone
And is operating in a debian stretch docker container.

Hotfix

A possibility to fix it temporarly is to restart the server or invalidate the caches.

We debugged this issue for quite some time and are using a hotfix, that stops caching from
get_forgotten_rooms_for_user in roommember.py by removing following line:
https://github.com/matrix-org/synapse/blob/9c1b83b0078aa9cc1bb902e14d3f7302625ba099/synapse/storage/data_stores/main/roommember.py#L797