Hi,
Thank you for the details, it helps a lot!
The issue is being caused by a 3rd party script, but I am not sure which one exactly. I am suspecting the navigation menu handler, as all of the plugin elements are being modified, classes are being prefixed with the “sidr-class-” prefix, like so: https://i.imgur.com/9QPsB9W.png
That basically disables the styling, as these classes are not defined in the stylesheets.
I would recommend asking the theme developer about this, or perhaps the nav menu plugin developer (if you use any). They should be able to suggest to disable this behaviour.