There is a bug with Isobel (or maybe other companions too) when she is using a ranged weapon, like the bow, and when she does not have a clear line of sight to the target.
If Isobel stands on the side of some stairs, or just around the corner in a house or a cave, where she can't se her target, she will just twitch instead of moving so she can fire.
The twitch is like she aims the bow but then immediately stops (since she cant see the target) and then she aims the bow again.. and so on.. over and over..
There is no attempt from her to move a little so she can get a clear shot, she is just twitching.
This have happen several times now..