The bug

When skeletons freeze, they convert to strays. This also applies to the jockey variants, however, instead of dismounting, the strays maintain their jockey status, allowing for them to ride skeleton horses and spiders. This is unintentional, due to MC-109415 being marked as WAI. Relates to MC-200418.

How to reproduce

1. Find or summon a skeleton riding a spider (or skeleton horse)
2. Dispense a powder snow bucket onto their head (make sure not to freeze the bottom mob)
3. Wait, eventually the skeleton will convert to a stray and still be riding the spider/skeleton horse