When you are riding a horse in a pre-1.11 world, then upgrade the world to 1.11, the horse disappears and you just fall on the ground.

Horses that are not ridden are converted properly (according to the type like donkey, skeleton, etc) but when ridden, the old id "EntityHorse" is not interpreted correctly, and the RootVehicle tag in the player data disappears.

[14:19:36 INFO]: UUID of player youtakun is ############################
[14:19:36 WARN]: Skipping Entity with id minecraft:entityhorse
[14:19:36 INFO]: youtakun[/###.###.###.###:51485] logged in with entity id 166384 at ([world]303.5373665449841, 70.8500000178814, 704.1786880611282)

When I use NBTExplorer on the (backed up) playerdata, change the 'id' tag from "EntityHorse" to "horse" or "donkey" (depending on if there's an 'Items' tag), and log in, I'm correctly riding my horse.

It seems a simple case of overlooking, that horses in the world are converted but horses in players are not.