Steps to reproduce the issue

Using Cassiopeia create an own css file with definitions to be used in the editor, e.g. myeditor.css - you can make a copy of the editor.css file and add a definition like
p.red { color: red; }

Go to plugins -> TinyMCE. Enter the name of the file in the field "Custom CSS Classes".

Expected result

You should see the new class in the formats dropdown in TinyMCE when editing an article:

Actual result

The myeditor.css file is not loaded and TinyMCE loads the editor.min.css file from media/system/css instead

System information (as much as possible)

Tested in Joomla 4.4, 5.0 and 5.1

Additional comments

If you rename myeditor.css in myeditor.min.css it will works