Messages sent from verified session marked as un-verified

*This issue was originally created by [**@alistair23**](https://github.com/alistair23) at .*

### Background information

- **Dendrite version or git SHA**: 037ff4fb2364cbee7a3a9a856cd2a711f5398682
- **Monolith or Polylith?**: Monolith
- **SQLite3 or Postgres?**: Postgres
- **Running in Docker?**: No
- **`go version`**: 1.15.6

### Description

I setup cross signing in Element and I can chat to others via encrypted rooms with everything working correctly.

I then sign in on the lastest build of Nheko. I then manually verify the Nheko device in Element so that it is treated as verified.

When I send a message from Nheko it still flags to other users as sent from an unverified session.

Looking at the `device_id` in the message I see it was sent from my verified Nheko instance.

I don't see anything interesting in the log to indicate an error

### Steps to reproduce

- Verify a Nheko instance
- Send an encrypted message.