I just encountered this bug where Millhouse Manastorm’s battlecry does not affect drawn spells while playing in the Blackrock Mountain Mage Class Challenge mode. Millhouse’s battlecry should have affected my drawn Unstable Portal, but the mana cost was still 2 while the spells already in my hand have been reduced to 0.
EDIT: I realized that I had Nerubian Unraveler out on the field, so I think that might have something to do with this as well. Regardless, I think the cost of the drawn Unstable Portal should still be 0.
See hyperlink for video. https://imgur.com/a/WnfUtaF
You’re right, it is Nerubian Unraveler. He has what you could say a “false” aura. His effect isn’t continuous, but kicks in when you add a spell to your hand. So, when Millhouse is played, all spells are set to zero, but when a spell is added to your hand, Unraveler says it has to cost 2 more.
Order of effects is important. Millhouse’s Aura caused all your spells to cost 0, but when Unstable Portal was drawn, Nerubian Unraveler increased the cost to 2.
Another example is the one given in the wiki regarding the interaction between Millhouse and Loatheb.
If Loatheb and Millhouse Manastorm are both played in a turn, their effects will happen in order: if Loatheb is played followed by Millhouse Manastorm, all enemy spells will cost 0 mana; if Millhouse Manastorm is played followed by Loatheb, all enemy spells will cost 5 mana.