when riding a horse under a tree it despawns

I found this very odd and mildly upsetting bug when I was heading to put slime blocks at spawn after the common dropping world height glitch happened.

Anyways I lost three mounts to this bug: a mule named thomas, a donkey, and this black stallion. I would just like to know what could be a possible cause thank you.

 

the spot of concern in the images is y:64, also there are no sounds being created by the mob so I have to assume they are deleted