one of my favorite things to do in survival mode is get a nice farm going with two of each pet/variant. so 8 llamas (2 of each colour) two donkeys etc... and all these animals i nametag because i have 100s from my afk fishing farm. but for some reason, ill have the odd mob randomly vanish. recently one donkey and a llama, but its been horses, dogs, cats etc... and i have no idea why this is happening because they're name tagged, im in survival so no debugging/killing and i dont have any auto farms with 1000s of mobs in the same area. so why? why are they going missing? if its a bug im going to have to quite this game for a while because this just tops off the fencing bug for me.
in the meantime im going to walk 2000 miles to get another brown llama since my other has vanished
What fencing bug? If you're referencing how mobs will glitch through fences, that's not a bug specifically with fences. The same exact issues exist for glass, leaves, and any other block that is considered transparent that can block mob movement, plus any non-transparent block (any block that a mob can suffocate in). The only way to prevent this from happening is to use water flows to push the mobs away from the walls/fences.
Anywho, nametags do not make animals invincible. They will still take damage from the usual sources (players accidentally swording them trying to hit something else, walking off ledges to take minor fall damage, getting attacked by mobs, suffocating in walls, etc), and since mob health does not naturally regenerate eventually they'll die.
If you are sure that the mob just disappeared entirely rather than dying or escaping, that's a good indication that your world has begun to get corrupt.
they're all being kept in stable styled builds. so a mixture of solid blocks and fences. but even animals that i've kept in just fences or just blocks have still gone missing/escaped. its something mojang can easily fix but its been an issue for years now. its a shame because this is the only thing that bothers me about this game. and its such a minor one