Playing a modded game and I noticed that my dog was gone. Took a while to find him, but he is in the Alex's Caves Biome and is spasming. He cannot move, his legs and body jitters rapidly and if you try to make him sit or stand, he spasms then returns to the previous animation.

If you mount him, you get permanently stuck and cannot move or dismount unless you teleport far enough away to unload the chunk.

I cannot figure out a cause or how to fix it.

This behaviour is neither ours nor in our control. Please report it to the other mod.
A possible fix meanwhile would be to /kill the Dog and then revive it using either /dog revive or a totem on an empty Dog Bed.