The PGCodeMirror is automatically enabled in defaults.conf to work with the PG problem editor. The necessary javaScript files for enabling CodeMirror have not been added to the File Manager page.
# This sets if the PG editor should use a js based "codemirror" editor or
# just a textarea
$options{PGCodeMirror} = 1;
# This sets if mathview is available on the PG editor for use as a minimal latex equation editor
$options{PGMathView} = 0;
(But you will need to transfer new features in localOverrides.conf.dist to your localOverrides.conf by hand since your local .conf files are not changed during updates. )
You might have to use Shift-Reload on pages you are currently editing to force the browser to reload the javaScript that makes CodeMirror work.