I'm getting the same symptom with TinyMCE Version: 5.10.9 (2023-11-15).

In my case too, the query is always an empty string. Haven't yet figured out a hack.

TinyMCE Version: 5.10.8 (2023-10-19) works fine, so defect was injected with TinyMCE v5.10.9.

I have a fix.

CAUSE: TinyMCE v5.10.9 and v6.7.3 explicitly changed how the "zero width no-break space" character(U+FEFF) is handled. Unfortunately, the mention plugin uses U+FEFF as a delimiter for the user-entered search text.

SOLUTION: Change the U+FEFF character to U+200B ("zero width space") in the 2 places it is used. Here's the affected lines.

OLD

# 67: '<span id="autocomplete-searchtext"><span class="dummy">\uFEFF</span></span>' +
#187: this.query = $.trim($(this.editor.getBody()).find('#autocomplete-searchtext').text()).replace('\ufeff', '');

NEW

# 67: '<span id="autocomplete-searchtext"><span class="dummy">\u200b</span></span>' +
#187: this.query = $.trim($(this.editor.getBody()).find('#autocomplete-searchtext').text()).replace('\u200b', '');

The line numbers reference the un-minimized plugin of course, so you'll also need to make equivalent changes in the minimized version. I'll open a pull request, although I'm not sure whether it'll ever be merged as this plugin seems to no longer be actively maintained.

I hope this helps others.

Yes, this easy fix has brought back the functionality. Thank you!

I hope this helps others.

It does, thank you @mellevsen !

Fixed the project to work with tinymce 6.x
Checkout my PR #96