I see this with mech mage too. I have the fireball spell(2 damage) that could kill a 5/2 or 6/1 in previous games but instead it puts up a 4/3 and ends turn, even though it had mirror images(2x shields) and it loses to the 5/2 and 6/1. It's a game that I should have won, except it played the highest minion I had instead of the right minion or spell.

Both in rush and normal default setups