Take off belt, rings, amulet. If then you insert any flat damage gems (Damage I slots) into your weapon and then use Unstoppable Momentum - you lose any corresponding damage from Bleeding Edge. For example if you insert 24-24 aether damage gem into a weapon and then turn on Unstoppable Momentum you will NOT get any aether damage for Bleeding Edge. Then if you turn that rune off - aether damage is there.
Flat damage from belt, amulet or rings works even with Unstoppable Momentum . So I think it is a bug.