Wrong default display name

### Steps to reproduce

1. Run a Matrix account with a default display name `
2. Change the display name in room `#x:mpl.org` using `/myroomnick ` using app.element.io
3. Install Element on iPhone
4. See `` preset as display name in the settings view of Element for iOS
5. Change it to ` in the settings view of Element for iOS
6. See the notification in `#x:mpl.org` that I have my nick changed

### What happened?

Expected behavior:

* the default display name set in app.element.io should also be the default display name in Element for iOS
* special room names should stay special only for those rooms where they have been set

Actual behavior:

I saw the special display name I set in two rooms using `/myroomnick ` being set in the settings view of Element for iOS right after installation from the app store

When I changed it back to the same name that is also the default display name in app.element.io, it changed the display name in both the rooms where I had altered to not being the default name. In every other room nothing was to be seen because there I already had the default display name.

Side note: the default display name is also the user name on the home server.

### Your phone model

iPhone SE

### Operating system version

iOS 14.8

### Application version

Element 1.5.2

### Homeserver

matrix.org

### Have you submitted a rageshake?

No