Cross-signing didn't work when a new session was added

### Description

A friend of mine (who was previously only using Element for iOS just installed Element on their Windows desktop (and verified his two sessions successfully). Previously we had verified each other, him from his iOS device and me from my Android device.

After my friend added his extra Windows session:
- His Windows session does not trust any of my sessions (grey shield for both)
- His iOS session does not trust me (red shield)
- His sessions do trust each other (green shield)
- My Android session and Linux session both trust his mobile session (green shield) and do not trust his Windows session (grey shield)
- My sessions all trust each other (green shield)

We both have cross signing enabled. Also, all of my clients have "Never send encrypted messages to unverified sessions from this session" enabled, but he can read all my messages (and the room is indeed encrypted).

How come my sessions do not trust his? How come his iOS session no longer trusts my sessions?

### Steps to reproduce

Unknown. I've submitted debug logs.

### Version information

- Me: Desktop, Linux, 1.7.29
- Me: Android, 1.1.8
- Him: Desktop, Windows, 1.7.30
- Him: iOS, 1.4.1

Start with the submitted debug logs and compare the cross-signing and session trust state across the listed Element clients and versions. Done means identifying why trust diverges after the Windows session is added and recording a reproducible path or confirmed fix; no source file or test is named in the issue.

Written by the indexing model from the issue text.