Users (mention macro) that have umlauts in their names doesn't get renamed properly

Issue Summary

When Collaborative Editing is ON, users mention for users that have umlauts in their names will not get updated accordingly if they get renamed.

Steps to Reproduce

1. Create a new User with umlauts in his name (NOT username), eg: "Ümlauts".
2. Mention the user with @mention on a page
3. Now Change the name of the user to something different, eg: "Umlauts"
4. Reload the page with the @mention and see the displayed name in the @mention is as expectet "Umlauts"
5. Edit the page (you will see in edit mode already the old name "Ümlauts"), change something (NOT the @mention) and update the page.
6. The @mention on the page gets updated to the old name "Ümlauts" and not "Umlauts"

Expected Results

The @mention on the page gets updated to the new name "Umlauts"

Actual Results

The @mention on the page still remains as "Ümlauts"

Workaround

This behavior is NOT replicable while Collaborative Editing is OFF.