Read the comment above yours; it's already been reported and is being handled under another issue. The bug may not be fixed yet, but they're already aware of it, so this discussion can close (to be continued at the other original issue)
I'm also playing on Nintendo Switch, and this has happened on every single load, in every single world, since about January 5th. It's making the game unplayable, because I can't reliably close the software and come back at a later time.
As noted by previous commenters, the last time I saved, I was in the bedroom of my house during the daytime with no monsters in the vicinity (at surface level, anyway, and all doors into the house were closed). Ceiling of the room was four blocks high. I was not standing on a transparent block nor had any transparent blocks above me, nor was there anything in the vicinity that could have caused a sudden and unexpected death. Upon loading the game, I was presented with a "you died" message and appeared to be at the top of a tree. Based on what J Unknown said, I think this was the world spawn point (I'd only created the world yesterday). I went back to the main menu without respawning, and loaded a copy of the game I'd made after saving (after googing this before, someone suggested this was a workaround). I was also dead on arrival there, in apparently the same location.
Fortunately in this case I'd only just finished building my home base and hadn't really collected much of anything yet, but I have no interest in playing the game again until I know this is resolved. As others have said, it's very disappointing to get to minecraft's equivalent of "maximum level" and be forced to start from scratch through no fault of your own.
I've spent the vast majority of my time so far in one area in a taiga region building a base, and nothing yet has spawned; occasionally I've made a "day trip" to a nearby plains biome, staying there nearly sunrise to sunset. All told, this is probably around 10+ hours of game play, with nothing spawned.
This is very inconsistent from Bedrock, which I've played in the past; in that version, while I wouldn't expect to be surrounded by chickens and whatnot during one of those "day trips", I'd at least expect to see a handful of passive mobs somewhere nearby.