Yep I know what you mean. Very annoying feature. It makes things tricky especially with Ults.
Not fun to cast a Destro Ult or DK Standard, only for it to disappear 2 seconds later just so that next wave of adds can rush you immediately after
I think it may be intended so that you can exit combat quickly after the last mob dies. And yes, I really don't want to wait 10s to get out of combat because I dropped LL just as the last mob was about to expire. But just as with everyone else, it also really annoys me with those respawning mob nodes! I wish they could fix those skills so they would work "properly" in both cases.
This was like that for like a year - zos just don't care. So adapt - use impulse for trash or something like this.