Nightmare, you run away because of the spell, IF the mob is still in your screen, DB will re-attack.
Bubble, two ways
1) If its SHIELDING then DB will attack what mob it CAN attack, so if 3 of the 4 are shielding it will attack the one that is NOT shielding. If there are NO other mobs around, it will stop attacking and go back to pathing, then if the mob is STILL in screen when shield goes down, will reattack, if the mob is not, will forget about it.
2) if its INVULNERABLE MINIONS, then DB will stick to the main guy only.
Hope that helps