Counterspell doesn't prevent damage from Djinn's Mark.
It grants Heal, Move Speed and Spirit, and is put on a Cooldown as if the effect was successfully parried, but doesn't protect a player from the damage.
Just tested and contrary to what you said it doesn't seem to proc the item at all (no heal etc). Either way sure seems like a bug (no clear reason why it shouldn't work).
I tested it with my brother the other day and it worked like described, but after reading your reply I decided to test it with a mimic and turns out the interaction is different depending on how the Djinn's Mark is triggered.
Counterspell's effects proc (but damage is still dealt) when max mark is applied.
Counterspell's effects do not proc when Mirage manually activates Djinn's Mark by pressing his "3" key or when the mark expires.
