Almost all rooms disappear after attempting sync on a poor internet connection

[Avi0n]

Steps to reproduce

1. Try an incremental sync on a poor connection
2. Either all rooms are missing from the Home and room selection screens, or only the latest room is still shown

Work around: Clearing the cache and re-syncing on a stable connection brings the rooms back.

Outcome

What did you expect?

For all the rooms to still be visible and selectable even if the sync didn't finish correctly on a poor connection.

What happened instead?

Either all rooms disappear, or only one the latest room remains.

Your phone model

iPhone X

Operating system version

iOS 15.4.1

Application version

1.8.14

Homeserver

Synapse 1.58.1

Will you send logs?

No

[pixlwave]

Thanks @avion540 this is very useful to have a lead for reproducing the bug. Closing as a duplicate of #5924.