I also just experienced this. I had it set up and working well.
I have password protected pages PWP. Then not sure what happened, but now it's no longer prompting me to go to the login page. Very odd.
I just see "Error loading page" ~ hmm... now I am wondering if this is caused because the content is NOT linked to the menu. In other words, I am going directly to content without an itemID, or without an itemID that's part of the mobile menu???
So I turn all content back to public... AND it works fine.
THEN, I reset those PWP back to Registered, and now it prompts me for the login form.
So it appears to be something with caching?? Odd, since I clear cache and cookies during my tests.
Is authentication is too complicated for the system??