Mogg Flunkies + Goblin Assault interaction bugged

Bug with Mogg Flunkies + Goblin Assault (+potentially other "must attack if able" cards)

Noticed this in an MM17 draft -- my opponent's only creature was Mogg Flunkies ("can't attack alone") and I had a Goblin Assault ("All Goblins must attack each turn if able"). Game would not progress past the attack step, as my opponent was forced to declare Flunkies as an attacker, but then could not move forward with them as the only attacker.

Correct interaction is that Mogg Flunkies cannot attack in this situation. (In a situation in which opponent controlled other creatures, Flunkies would have to attack if at least one other creature was declared as an attacker, but would not be forced to attack otherwise.)