I will try your suggestion, however all my followers are doing it. Grated lately I have been placing some followers in chairs to avoid the falling through foundation bug, but thralls that have been standing forever are doing it, and even ones that were freshly placed standing.

They do not react even if I am being hit by the boss right beside them. They will only react if they are directly hit.
Typically I can work around it by standing behind my thrall so that the boss/enemy hits them when they swing at me, but this is just a minor workaround since they will freeze right again after one attack sequence. It makes thralls - already nerfed into the ground - absolute trash.