I am facing a very weird behaviour:

Using the latest JCE Pro ( 2.9.83) + latest Joomla (5.2.5).

I've added my custom CSS to the editor (Profile > editor parameters > Typography > Editor Styles), it is overwriting as expected.

The problem is that JCE is using the following CSS (https://mysite.com/media/com_jce/editor/tinymce/themes/advanced/skins/modern/content.css?0878a38d79bd865bc0e2dba2d2f4ec3e=1&context=19&profile_id=6) and this stylesheet is changing the link colours of my content when I view it in the editor. (the styles responsible for that are these: body.mceContentReset a, body.mceContentReset a:link)

I placed an aditional CSS file to the editor styles, called "cms-editor.css", and I discovered that if I have the following definitions:

body {
color: #4C4B49;
}

It clears the overwritten link colours and it is now ok!

But the most interesting thing is that if I place the black colour:

body {
color: #000;
}

The link colours are overwritten (wrongly) again!

I had it in "auto", turning it off solved the issue.

This means that the "auto" option could misbehave, maybe you should take a look at that.