In that case, no, I have not experienced a similar problem. My thralls seem to be quite aggressively engaging the enemy. When I have two thralls at the same time, the other one may be slower to approach the enemy (one rushes at the enemy, while the other one “stalks”), but both attack and keep attacking until whichever nuisance they found is no longer moving.
Is your problem affecting only specific thralls, or does the same happen with all thralls? Maybe there’s a flaw in certain thralls’ AI rather than the whole?