A world my kids and I have been playing on for weeks, lots of buildings all over the place, small nether hub and lots of other work. After the 1.12.0 update I would see the saving icon in the corner stay for long periods of time but didn't think much about it. I went to the nether, built a nether portal, went into the overworld to build, then went through the nether portal again and found myself in solid netherrack since building the portal never saved. Then I built it again and went into the overworld and found none of my building had been saved either.  If I save and quit after this behavior starts it will go through the save messages until it gets the the one about "Don't turn your Switch off." then stops on that screen forever.  When I force it to quit and restart Minecraft all the most recent changes are lost and it loads back to a savepoint some time earlier. It feels like my client has become de-synced with a server and none of my writes to the server are saved so every time I go to a new area I read the server data without my changes, but this is all locally on my Switch.

The closest thing to a work around that I've found is if I stay in 'safe' chunks that have been loaded for a long time and do a "Save and Quit" every few minutes. If I travel too quickly I get de-synced and saving is broken. If I go too long without a "Save and Quit" I get de-synced and saving is broken. Sometimes I can get into or out of the nether safely, but most of the time that will cause de-synching and saving is broken. Sometimes I will get the Switch system message that the program has closed unexpectedly (which is frustrating) but most of the time it will allow me to merrily build away while it blithely pretends everything is ok and not save anything (which I find even more frustrating).