i have been having this problem for more than a year now. it does not matter if mobs are named, saddled, tamed or bred with. realm or normal world. put in enclosure away from chunk borders. they despawn constantly. i have lost a whole village worth of villagers on more than one occasion. many horses with saddles and nametags. its getting to the point where i no longer wish to play bedrock. and that would be a great shame.
i am also having this issue, im playing mc for windows 10 version 1.16.40 but also happens on mobile same version. also it doesnt matter if i play on realms or on a private world its just the same. hope they find a fix soon, its super annoying.
for me it just wont open at all, i get the loading circle above my curser but then it just fails to open, even if i open task manager it does not show as a running app. i can only play bedrock version by opening from the start menu. i cannot acces java at all. hope tey find a fix soon.