XTypo and Joomla 4

I use JCE and already have an active custom CSS file (editor.css)

Is the file editor.css the file from the old version of xtypo?

If the css file is the old one of Xtypo then simply replace it with the new one.
If it's your own custom styles, then simply click the plus sign in the right side of field to add additional custom css path

What joomla and PHP version do you have? I'm unable to replicate the issue at my end.

Btw,Is it possible for you to share temporary access to your administrator page so I can check it out directly? If you can, you can use the 'confidential' feature on this forum by clicking the lock icon in the editor toolbar, then enter the credentials between the tags. Only the support team will be able to see the credentials