I saved into a structure block a wither skeleton riding a polar bear. When I used a structure block to load the structure at another location, the polar bear was present, but the wither skeleton had vanished.

I expect this is a bug with how saving/loading entities is handled by structure blocks.

I've attached photos showing the polar bear riders from the saved structure, the structure block's load menu settings, and the polar bears lonely after being loaded in without riders.

Hope this can be fixed!