Bought horse standing still when whistle - BUG - PC

I am into “All That Glisters” main quest and I started to notice my horse doesn’t walk to me when I whistle.

Normally if my horse is within a certain range, he will try to walk to me. But recently he started to just stand still and ignore my whistle.

Although I could still make him teleport to me if I am too far away, if I’m too close, my horse won’t appear next to me and I have to find him myself. Or whistle again after I fast-travel to some where else.

This is kind of annoying as I already get used to my horse walking close to me when I am near.

I tried to reset the horse by buying another one, but it didn’t help.