[c10y] Sable seems to accept local invites on Continuwuity improperly

[monospacedmagic]

Describe the bug

After accepting a local invite on a Continuwuity homeserver using Sable with Sliding Sync enabled, it doesn't seem to join the space properly. Instead, it seems to be in a half-joined state. When I close Sable and reopen it again, it acts as if I had never accepted the invite, as I'm no longer in the space and the invite is once again in my inbox.

Reproduction

1. Using Sable, on a Continuwuity homeserver, accept a local invite to a space on the same homeserver.
2. Close Sable.
3. Reopen Sable.
4. Find the space you just joined gone from your list of spaces, and the invite back in your inbox.

Expected behavior

It joins the space properly.

Platform and versions

1. OS: Android 17 (GrapheneOS v2026071501)
2. Browser: Vanadium WebView v151.0.7922.29.0 (based on Chromium WebView)
3. Sable version: v1.20.1-nightly.2607240604
4. Homeserver: monospacedmagic.dev (running continuwuity:main (commit: 4727f07e26) via Podman)

Additional context

On Element X, joining those same spaces seems to work.

I've also created an issue on the Continuwuity repository, but can no longer reproduce the issues described in this issue with other clients after leaving the spaces that I've "half-joined" with Sable previously, so I'm inclined to believe that this is an issue specific to Sable being used on a Continuwuity homeserver. Even after I've joined the spaces seemingly properly on Element X, they still don't show up on Sable.

[monospacedmagic]

For the record: @eleboucher asked me if I had "failed to handle PDU effects of incoming PDU" in my homeserver's logs, I didn't.

[monospacedmagic]

For the record, PR 1284 did largely alleviate the issue, but not fully. I'm guessing for that the issue on the backend side of things would also need to be fixed, presumably by PR continuwuity#2039.