This is most probably a bug. Every time Imprison wears off I get in combat with the CC-ed mob and the whole pack, if the mob is a part of a group. This happened to me a few times when we were trying a skip in Mists of Tirna Scithe, but it is also bugged outside of dungeons, I tried a few different mobs and zones.
I am not in combat before or during the Imprison spell, I get in combat after it wears off.
Is this a known issue?
If someone else gets in combat with the group (by walking too close, for example), you’ll get in combat too. Imprison doesn’t mean you can walk right through the mob. Your party members will still need to be careful to avoid getting in combat.
If any of them CCs it too, with a spell that does put them in combat, you will also get in combat.
This is most likely due to getting too close.
Some CCs reduce the aggro-radius like a Rogue’s Sap, but a Paladin’s Repentence not so much. So some CCs are better to use for skips than others. No clue about Imprison tho.
It happened to me. Figured out it was a conduit that “slows” once the imprison is off, " Demonic Parole" that causes the issue. Removing it fixed it.