Verified session is shown as 'Not trusted'

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

### Background information

- **Dendrite version or git SHA**: /etc/dendrite # 0.13.7+14a6c10
- **SQLite3 or Postgres?**: postgres
- **Running in Docker?**: yes
- **`go version`**:
- **Client used (if applicable)**: Any client

### Description

- **What** is the problem:
Some verified sessions shown as 'Not Trusted' to others if contact verified after session
- **Who** is affected:
Any who verify my contact after I've added new session
- **How** is this bug manifesting:
Your contact shown as not trusted to others when then verify you
- **When** did this first appear:
When I register new session with other device after contact verification

session AG3_a329 is Verified. Marked as green and all seems nice.

session AG3_a329 shown as not trusted

Refreshing clients does not help. Problem is reproduces on app.element.io

### Steps to reproduce

- create test account on homeserver
- create test account on matrix.org
- verify each other
- register new session in homesever
- session of test user shown as 'Not trusted' on matrix.org

Start by reproducing the listed two-account flow on Dendrite 0.13.7+14a6c10 with Postgres, then trace the server handling of device verification and newly registered sessions. Done means a session added after contact verification is shown as trusted to the other user, including when tested through app.element.io.

Written by the indexing model from the issue text.