It happens with any link I really don't know what make it happens. I use "Inverted listed only".

It magically fix alone if you wait for sometime like one minute using a another tab than back to the protected tab or reload and fixed.

I get it randomly when I create a new tab search from Firefox home page, open a google result, then page protected. I tried it again and the problem don't happened. I back to this tab I am writing here at github then protected again. Just randomly. I am unable to reproduce.

Do you use any extension that modify the tab's URL?

No. That happens with less frequency today, probably because it was a bug in old Firefoxes. Today I see the dark and light mode switching OK, but the icon V and X before the site name(domain) doesn't change in Dark Reader interface sometimes(random).