I use Joomla 3.9.21 and JCE 2.8.17 PRO and tested this issue in the most recent version of IE Edge and Chrome.
I noticed that the layout of the content in the backend editor wasn't how it looks in the frontend. It looked the same before.
In the Global Configuration of JCE at "Editor Styles" I have "Template CSS File" selected.
If I disable "Compress CSS" the content in the backend editor works good.
If I enable "Compress CSS" it doesn't work.
Is this a bug?