My experience was similar, I figured out that it was the water/mud if I lead with anyone besides him, he would run the long way around to get behind the cub. If you lead with Astarion and force him to cross the stream in front of the Owlbear he would behave normally. For some reason Sunlight doesn't damage him, water does so his pathing would be to avoid walking through the water.
Astarion is a Vampire - he goes into detail about it once you let him feed off of you
