user_parter removes other users from room when deactivation endpoint is called

[matrixbot]

This issue has been migrated from #16582.

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Description

Whenever I call the deactivate (+erase) endpoint on synapse (this case for userA) it kept removing another user (userB) from all rooms.
Today this happened for a second user (userC). The notable difference here is that userB had left one of the rooms on their own. C hasn't so far.

Steps to reproduce

• Deactivate+Erase userA
• Watch public rooms
• See userA unsetting their display name and leaving
• See userB and userC leaving all public rooms they were in

Homeserver

Selfhostes

Synapse Version

1.94.0

Installation Method

Docker (matrixdotorg/synapse)

Database

single postgres server, no restore from backup

Workers

Single process

Platform

Debian 11 with docker

Configuration

Presence and retention are enabled in the affected public rooms.
The public rooms have a 48h retention policy for about 1.5 years.

The homeserver does not federate.

Relevant log output

2023-10-31 09:18:04,843 - synapse.handlers.deactivate_account - 158 - INFO - POST-911131 - Marking @userA:myhomeserver as erased
2023-10-31 09:18:04,845 - synapse.federation.sender - 592 - INFO - process_event_queue_for_federation-1377 - Unexpectedly did not have cached prev group for $oxitnVVee-cjwUyKUZgPf1iHCbQ0kYYvXbov3uoZXyU
2023-10-31 09:18:04,853 - synapse.handlers.deactivate_account - 234 - INFO - user_parter_loop-5 - Starting user parter
2023-10-31 09:18:04,861 - synapse.handlers.deactivate_account - 240 - INFO - user_parter_loop-5 - User parter parting '@userB:myhomeserver'
2023-10-31 09:18:04,873 - synapse.handlers.deactivate_account - 254 - INFO - user_parter_loop-5 - User parter parting '@userB:myhomeserver' from '!wmwqhJvBlcVtlMzYXp:myhomeserver'
2023-10-31 09:18:05,119 - synapse.handlers.deactivate_account - 254 - INFO - user_parter_loop-5 - User parter parting '@userB:myhomeserver' from '!WCxYbgOzUJowbPisiS:myhomeserver'
2023-10-31 09:18:05,127 - synapse.federation.sender - 592 - INFO - process_event_queue_for_federation-1378 - Unexpectedly did not have cached prev group for $S4RKH353eV6CNCxkP3aVVVreIu6qLYmuqZEhwV9fhDE
2023-10-31 09:18:05,338 - synapse.handlers.deactivate_account - 243 - INFO - user_parter_loop-5 - User parter finished parting '@userB:myhomeserver'
2023-10-31 09:18:05,339 - synapse.federation.sender - 592 - INFO - process_event_queue_for_federation-1379 - Unexpectedly did not have cached prev group for $mJL1JXe8swj8vFf0MZLdXvwx4UZxRf1YyAw1Q42L-yw
2023-10-31 09:18:05,343 - synapse.handlers.deactivate_account - 240 - INFO - user_parter_loop-5 - User parter parting '@userC:myhomeserver'
2023-10-31 09:18:05,352 - synapse.handlers.deactivate_account - 254 - INFO - user_parter_loop-5 - User parter parting '@userC:myhomeserver' from '!wmwqhJvBlcVtlMzYXp:myhomeserver'
2023-10-31 09:18:05,553 - synapse.handlers.deactivate_account - 254 - INFO - user_parter_loop-5 - User parter parting '@userC:myhomeserver' from '!WfkDZztWcIEzOOmRNW:myhomeserver'

Anything else that would be useful to know?

No response