Assuming the situation:
Eder (with an active Defender skill) is tanking 3 foes in close-combat. Eder has really strong defenses, so mobs can't even scratch him. At some point mobs' AI decides that Eder is not so attractive target for them, therefore it is time to disengage Eder. Following is my guessing: there is some sort of internal damage threshold value, which has to be applied to the target for every X seconds in order to continue engaging this target. Thus, because mobs can't do this - they are switching to the easier prey. You may already know what should happen next: short-lasting Benny Hill style chasing, when Eder will unsuccessfully try to re-engage at least some of the foes, then mobs are getting to chosen party-member and effectively one- two-shot him/her because it is Death March.
Well, it is actually not very usual situation for Eder because fighters hasn't got such superb defenses like paladins, but anyway - it may happen even with a fighter.
The problem is: it's basically too painless to get free from a tank's engagement. Tank can't really punish for this, because usually she is an worst damage-dealer in your party. Which leads to the question: doesn't tank need an instrument to prevent/resolve such situations in some way? For example, allow tank to apply a brief Hobbled effect on the disengaging targets (by choosing a separate talent if it's needed).
What do you think?