This is still an issue when playing on the switch in the latest update 1.16.210.
I was able to play on realms but still got this loading bar when opening menus and chests.
Aside from this bug, i can see other improvements such as the responsiveness of the UI is overall much better.
Login and startup feel slightly quicker but I have not timed it yet.
On my son's switch, it can take 30 minutes or more to load.
On my switch, it loads quickly.
The difference is my son has 260 worlds, each one of these is only a few Mb. In comparison, I have about 10 worlds of varying sizes.
My assumption is that the worlds (or part of the worlds) are being loaded before the game starts.
I think its more to do with the number of worlds, not the size of them.
I have timed this on the switch, when I connect to a friend’s game, their switch freezes after 10 mins, then kicks me out.
i can rejoin but it will freeze and kick me out another 10 minutes later.
This has only happened since the 1.18.30 update, it was very stable before.