Playing on a Realm with 4 other people for about a year now.
All of us had named horses, and over the course of a few months they have just popped out of existence one by one in various areas of the map. Can be there, and then within 5 minutes they just pop out of existence.
At one point, we even had an entire farm of animals decimated overnight without any hostile mobs attacking them - well lit and penned in.
This is a really, really annoying issue, and it's borderline impossible to mitigate. We actually pay an ongoing cost for a server, and the fact we can't rely on the game to correctly maintain things like this (plus it's been ongoing for at least a year), it's almost impossible to remember to tie up your horse on a divisible number of blocks (because you can't easily identify chunk borders) and we can't switch to Java Edition because half of us play on PS4/PS5, one on Xbox and two on Windows. 😞
I think that's a new moon - it should be more full on the next day.
https://minecraft.fandom.com/wiki/Moon#Phases