When the Skeleton rider of a Spider Jockey or Skeleton Horseman is put inside Powder Snow, it dismounts upon converting to a Stray.

How to replicate:

Spawn a Spider Jockey or Skeleton Horseman

Trap it in a Minecart to keep it still

Put a block of Powder Snow on the Skeleton's head

Wait.

Expected Result:

The Skeleton converts to a Stray and remains on it's mount.

Actual Result:

The Skeleton converts to a Stray and then dismounts.