This might be a duplicate or variant of MCPE-34662, but please read through as there might be new/different information available.

When starting Minecraft for the first time it signs in to the Nintendo Account, and then the Microsoft account, and I'm able to play and record achievements. The main menu shows my XLB tag  as expected. See attached video, "part I". In-game I can now view my achievement, and the menu screen before shows the GT in top line, see attached file "WorkingConnection.png".

If now I go out of the game into the Nintendo Switch main menu, and look at news and enter the sleep mode, something happens with the Microsoft Account link. (The same occurs if I just leave the game running, and it times out and goes into sleep mode by itself). The next video, "part II", is a recording of what happens when I re-enter the game where I've now lost the link to the Microsoft Account.

Achievements and so on are not accessible from either in-game, nor from the main menu.  I also tried to do another sign in to the Nintendo account, but nothing happens. Still disconnected.