New room was "untrusted" and user could not send messages

### Steps to reproduce

I just finished a debugging session with user A. They were unable to send messages in a newly-created room, having been invited by user B. We resolved it by both users leaving the room and creating a new one. The old room did not disappear for user A, so we did clear cache and reload, and it was gone. The new room worked fine.

1. User B invited user A to a DM
2. User A accepted
3. User B sent a message.

### Outcome

#### What did you expect?

To be able to send and receive messages

#### What happened instead?

User A could not see user B's messages, and the room info said "Untrusted".

User A was not able to send messages - when the tried to send a spinner appeared next to the message, and eventually an error about failing to send appeared, offering to delete or retry.

### Operating system

Windows

### Browser information

Chrome

### URL for webapp

_No response_

### Application version

_No response_

### Homeserver

_No response_

### Will you send logs?

Yes

Start by reproducing the invitation flow in Windows Chrome: create a DM, invite and accept a user, then check the room's "Untrusted" state and failed message send. Gather the promised logs and missing application, URL, and homeserver details; done means invited users can see and exchange messages in a newly created room.

Written by the indexing model from the issue text.