Synapse sends events over /sync in rooms that are mid-faster-join, to clients that don't support membership lazy-loading

[TheArcaneBrony]

Description

Synapse appears to send new events down sync for clients that don't support lazy loading.
This leads to clients having rooms that contain no state events.

Example in nheko:

Steps to reproduce

• Join one or more "large" rooms (~150-200 in my case)
• Notice new "empty rooms" appearing in client as people send messages, join/leave, ...

Homeserver

rory.gay

Synapse Version

1.114.0

Installation Method

Other (please mention below)

Database

PostgreSQL 16.4, single db, no portdb/restores

Workers

Multiple workers

Platform

NixOS, VM with 256GiB of RAM and 14 threads off an AMD Epyc 7262
(Synapse from nixpkgs/master)

Configuration

Presence: on
Message retention: not configured
Experimental features: MSCs 2815, 3026, 3266, 3916, 3823, 4151

Relevant log output

None, sync workers only appear to log "Processed request" (CS-API) and "Received response" (replication), no warnings emitted (INFO loglevel)

Anything else that would be useful to know?

This is a single person homeserver! This may not be reproducible if the homeserver already has a local copy of the room.

Please probe me on Matrix (matrix:u/emma:rory.gay) if more information is needed, as I don't actively monitor GitHub or email.