tab completion treats search for @r to include users starting m

### Description

Typing `@r` in a channel does not tab complete to users with "r" at the start of their name.

I'm fairly sure this used to just show users which matched 'r'

### Steps to reproduce

Type `@r` in the message box:


Get these options:


Describe how what happens differs from what you expected.

I would expect that matmaul no 'R' in display name or mxid, matthew and manu (an R in their mxID only: 'matrRix.org') are not offered as suggestions, and instead richvdh with R at the start of their name, to be among the first in the suggestions.

Beyond these first few users which are definitely wrong, the remaining users all start R or M as well - it's like the two letters are confused with each other.

Logs being sent: yes

### Version information

- **Platform**: web

For the web app:

- **Browser**: Firefox 85.0.1
- **OS**: ubuntu
- **URL**: app.element.io