backstory:

I wanted to structure block clone a house

What I expected: 

mobs in the structure block previews (when you right click a structure block it shows you a preview of the structure's area) would appear with their bodies the right way up

what happened:

A pig that walked into the previewed area had its body pointing vertical. The feet and head were the right way, but the body was vertical with the underside facing the head. This also happened with a cow. However, when the cow stepped into the area, the pig reverted back to its regular format. when I summoned a cow into the area, the first cow reverted to its regular format and the new cow went the wrong way. This is quite strange. In some other cases, the pigs head has been facing upward, then it suddenly switched to the glitched body. I think that one body part in one mob in the preview zone is glitched. I've only noticed it since 1.16.10.

Recreation:

1: get a structure block with 

/give @s structure_block

2: set it to a large enough area.

3: go to an area in the preview zone

4: type 

/summon pig

5: quickly open the structure block. 

the pig's body (or other body part) should be vertical in that picture you see of the preview zone

6: (optional) summon a cow to the preview zone

7: (optional) look at the structure block's preview zone by right clicking the structure block.

one of the mobs would have reverted back to its original format. 

If this bug doesn't happen, repeat steps 4 and 5 until it happens. It will eventually.

thank you for taking the time to read this.