Seems like lifelink could do some code revision. This bug should be relatively easy to duplicate and fix.
Here’s how to recreate the bug:
You - any creature
Opponent - any creature with lifelink
Step 1: Opponent’s lifelink creature gets vigilance or defender
Step 2: Before you attack, you put Enshrouding Mist on your creature.
Step 3: Opponent gains life through lifelink even though no combat damage is dealt by lifelinked creature because of Enshrouding Mist