This absolutely happens on Reddit's mobile site; a little popup-overlay/link reading "Continue on the app!" blocks the content you're reading like you've run out of free Quora questions, can't be closed, and persists on reload.
Maybe they don't do that, but for sure they log me out almost every day from Firefox/Android and I'm willing to bet dollars to peanuts they don't do that on the app.
I even have freaking 2FA but they still don't remember the device and ask me every time for the code.
i think that happens because of cookies, if they're cleared you get logged out. do you have settings on firefox that do that? i don't use it so can't tell you.
Try it on Firefox. Plenty of things don't happen on Chrome because that's all devs test. That and Safari iOS.
Which does make things less of an evil intent and more of a "if our laziness pushes our users to where we make money, that's an unfortunate side effect".
this has never happened to me in 5+ years of using reddit