Encrypted message sent by users who then leave are lost

### Steps to reproduce

1. Put your phone away so that Element iOS is not running
2. Some user (not you) sends messages in an encrypted room
3. That user then leaves the room
4. Check the room in Element iOS

### Outcome

#### What did you expect?

I expected to be able to decrypted and read the messages from the user who left after sending them, just like I can on Element Web.

#### What happened instead?

The recent messages from the user who left after sending them all show unable to decrypt errors on iOS.

### Your phone model

iPhone 8 Plus

### Operating system version

iOS 15.2.1

### Application version

Element 1.6.13

### Homeserver

matrix.org

### Will you send logs?

No

No files, tests, or entry points are mentioned. Start by reproducing the encrypted-room scenario on Element iOS 1.6.13 or a current build and compare it with Element Web. Done means messages sent before the sender leaves can be decrypted and read in the iOS app.

Written by the indexing model from the issue text.