Hide sidebar switch disappears on mobile device

I found the option to hide the sidebar in the user configuration page. This works fine on my computer.

I have (had) the same option on my phone. The hide sidebar switch was deactivated (although the sidebar hides automatically on the phone), so I was curious what would happen when I activate it.
Nothing happened. So I switched it off again and the switch and it’s explantation immediately disappeared.

Is this normal? Could someone also try it out?
I do unterstand that this switch makes no sense on a mobile, but why is it here at the first place?

You’re missing a vital bit of information.

iOS or Android?

Official iOS app and also in Safari.

BTW: I just figured out that resetting the Frontend Cache brings back the button. But I can reproduce that the switch disappears after switching on/off.
Is it a bug?

Yes, i had exactly the same experience and also had to do the cache reset.
must be a bug

Just had the same experience on Android.