new chunks do not load while riding horse

I was exploring my world on a horse I had recently found, and noticed that chunks weren't being loaded as I approached them (see on_horse.png). I sat still on my horse for several seconds, but still the chunks wouldn't load.

But, when I got off the horse, the chunks started loading immediately (see off_horse.png).

seed 1129634059383932221
X=-2621, Y=66, Z=1297