If it's 'ensnaring strike' , they take damage per round while ensnared, but can stil attempt to free themselves each round and the spell is then over...so no idea what this is. 5E doesn't really do the old D&D thing of 1 save and your ^%$ed for the rest of the combat - you generally have a chance to save every round for a spell effect. Sounds like 'ensnaring strike'...and a bug.
