Failed to verify signature from own device.

# Description

I have two devices (Element Desktop/Android) set up to use encryption. It mostly seems to work fine. But in one room messages send from Android appear with a white shield: "The authenticity of this encrypted message can't be guaranteed on this device." The other party sees an "unable to decrypt". Messages send from Desktop look fine both on Android and for the other party.
Any ideas what might be wrong? Tips for troubleshooting?

Some further information:

- Both of my devices show eachother as verified.
- I have 4 additional "devices", double-puppeting bridges logged in via matrix-synapse-shared-secret-auth. But none of those have created cryptographic sessions.
- I think I might have seen this "The authenticity […] can't be guaranteed on this device."-shield somewhere else, too. But right now I can reliably reproduce it in this one room. Other rooms I tested seem to work fine.
- The m.direct for this room changed as previously it was a DM with a Discord-puppet, now with a real Matrix-user.
- Both of my devices have at some point imported the same key-backup (after migrating from my matrix.org MXID). Could that have mixed up their sessions or something?

### Steps to reproduce

- Send Message from Element-Android into this room.
- Correctly recieve message on Element-Desktop.
- Notice the white shield next to the message and read the warning.

I can't reproduce this issue by creating a new room or posting into another similiar room.

I would expect my messages to be correctly signed and encrypted to all participants in the room.

Log: sent from Android. Creating this issue so that I'm able to send from Desktop.

### Version information

- **Platform**:
- Element Android 1.0.9 F-Droid
- Element Desktop 1.7.13 Flatpak on GNU Guix/Linux (orig.: 1.7.12, but problem persists on .13)