Ok, that has annoyed me for a long time, time to post about it:
When cleric has mark on tank, both tank and cleric are locked into combat for a while after the encounter ends. This seems to happen in close to 100% of the encounters in my case.
This is very annoying both as tank and cleric - prevents opening clickables like doors and chests.
This *could* be related to Healing Word ticking, but I never really verified that. I am mostly using Exaltation-boosted Healing Word for my heals.