This reply has been accepted as the best answer.
Good news cleanup/reset worked and the CSS file is no longer corrupted, and so I can now clearly see what breaks it
Just before the TEST section title there is an invalid rule
input,textarea (line-heigh:0.5) /* TEST fonts and custom CSS file */
Which uses parenthesis () instead of curly brackets{} (edit: and a typo on height)
This is what breaks the CSS file, so nothing after that is taken into account.
So now it is about finding where this bad rule comes from.
I suspect a CSS+ box and I am going to try and find which one generates that rule.
You can search on your end. If we can’t find it, we will have to use the WordPress CSS box for your custom CSS