That is just so interesting! I wonder why that happens. Have you figured out how to access that class again to edit styling when this happens?
Additional info/side note: I went back to the project window (after not touching anything for an hour or so) and also not refreshing… and the class has now re-appeared as expected in the selector field again (at least for the .mobile-nav-link but I haven’t checked the other missing classes yet)
So since it’s reappeared, I’m able to see the blue dot indicating styling present on that element so I have no idea why it disappeared! 
mind-boggling indeed