I've learnt the hard way on Minecraft to never trust a pack-Donkey with valuables.
I managed to tame two Donkeys while I was renovating my world's NPC village (which conveniently is in a Plains biome) both of which died.
- The first one drowned because I ran it into the water and it got stuck on a dirt bridge one block wide, and it was too stupid to go in any direction so it wouldn't drown
- I leashed up the second to a fence post which was near a cactus I planted for easy trash disposal while I was building elsewhere, and didn't count on it being stupid enough to get killed by said cactus. All that was left was a leash knot and a missing donkey. Everything it dropped must've either gotten destroyed or despawned by the time I'd noticed. Thankfully everything but the saddle is replacable but it had 20 blocks of Redstone and two stacks of Rails on it. Poof. Gone, just like that. I'm glad I didn't take my entire supply of Redstone but the rails are going to require so much Iron Golem farming to replace.
Probably best using them to hoard items easier to replace, like Villager trade items like raw meat and Wheat, or easily reproducable items like Pumpkins and Cobblestone (via a cobblestone generator)