You lose melee engagement when using the Flames of Devotion Paladin class ability (which is melee ability).
Easy enough to reproduce, but most obvious when you have the Hold The Line talent and a Guarding (+1 engagement) weapon.
1. Combat log will show your character engaged on three enemies. You also see the green line indicator to your engaged targets.
2. Use Flames of Devotion on a melee target. ALL green engagement lines disappear for a few seconds (more than long enough for enemies to charge off to your backline). After the attack resolves, you you'll re-engage any targets still next to you.
If this is intentional, then it's inconsistent with other melee abilities (Fighter Knockdown maintains engagement).
