Authenticity of some encrypted messages inexplicably can't be guaranteed

### Steps to reproduce

I'm not sure how to properly reproduce this, since it seemed to just start happening (retroactively marking old messages with grey shields as I scrolled up as well).

1. Open E2EE room
2. Notice grey shield on seemingly random incoming messages
- I haven't found a real pattern yet.
- Messages from users on the same homeserver as me seem fine so far. At least one other single-user homeserver seems to also be consistently good.
- So far, I have seen messages from `beeper.com`, `matrix.org`, and (most?) single-user homeservers get inexplicable grey shields.
- In all cases, the same messages validate correctly on my laptop (running Element Web).
4. Scroll up to past messages which used to be just fine. See grey everywhere.
- Everywhere. Even messages I'm pretty sure I sent from this phone, without signing out from said phone the meantime.
5. Try to clear cache and reload. Same results.

All my sessions are, as far as I can tell, verified correctly. Both my phone and laptop display all of my sessions as verified. I have submitted a rageshake with as much information as a rageshake is allowed to contain and a screenshot of some affected messages (though I think maybe I should've filed this first so I could include a reference... whoops).

I doubt this is a homeserver problem, given that my laptop is behaving correctly even after clearing cache and reloading on it.

### Outcome

#### What did you expect?
Incoming messages probably shouldn't have issues. Old messages almost certainly shouldn't have issues.

#### What happened instead?
see description above

### Your phone model

Pixel 6

### Operating system version

Android 13

### Application version and app store

Element version 1.6.2 [40106022] (G-9f797e49)

### Homeserver

boiler.social (Synapse 1.85.2)

### Will you send logs?

Yes

### Are you willing to provide a PR?

No