Cannot decrypt new messages while key backup is running

### Steps to reproduce

1. Load a (suitably sized) key backup. Mine has 82k entries.
2. Have a message be sent to you in an encrypted room.

### Outcome

#### What did you expect?

The message to be decrypted immediately.

#### What happened instead?

The message was not rendered at all. There isn't even an error. The only reason I know a message has been sent to me is that I can see a `m.room.encrypted` message with Hidden Events turned on, and that I got a notification for the room.

### Operating system

Arch Linux / Wayland w/ Sway

### Application version

Element Nightly version: 2022011001 Olm version: 3.2.8

### How did you install the app?

Arch package

### Homeserver

half-shot.uk

### Will you send logs?

Yes