I don't know is it the right place to post this topic, but I couldn't find a "bug" session.
tl;dr: the game thought that Warbot + magnetized Zilliax is a single minion and applied enemy hero's ability (-2 attack to all enemy minions) twice (-4 attack in total, instead of -2).
Well, the boss (Haro Setting-sun) was pretty strong for my Warrior's adventure deck, but I captured some strange behaviour with a magnetic mech.
I put Warbot (mech 1-3) onto the battlefield.
He has +1+1 (because I added him with help of a Tavern Encounter: add all 4 enemies, they have +1+1 characteristics), so 2-4.
Then I used a hero power (Invigorating Brew: deal 1 dmg, add 2 attacks to a minion). The Warbot has 4-3 characteristics. And because of his card text, 5-3.
Yes, Haro Setting-sun's ability, so 3-3 characteristics.
Then I combined Warbot with Zilliax (3-2, everybody knows).
So, the final characteristics of a Warbot+Zilliax must be 6-5.
However, I guess the ability of enemy hero is applied to Zilliax, too (even he is not a separate minion), so 4-5. :(
But wait: Magnetic is an ability exclusive to certain Mech minions which allows multiple minions to be merged together. Playing a Magnetic minion to the left of an existing Mech will automatically cause the two minions' stats and card text to be combined into a single minion. Source: hearthstone.gamepedia.com
In conclusion, 1 separate minion on the battlefield (Warbot+Zilliax magnetized), 2 cards played (Warbot, Zilliax), Haro Setting-sun's ability is applied to a minion, but actually to both Warbot and Zilliax.
I wish you not to face such a situation!
I want to understand if the problem exists, and to report Blizzard's team about it. But if everything's alright and I made a mistake somewhere, please tell me! Thanks for your attention!
P.S. In the attachments below you can see an image with a 4-5 Warbot as I described in this topic. On the next image you can see the next turn with a Warbot+Zilliax healed with passive hero ability (First Aid Kit), -1 attack + 1 hp, so new characteristics is 3-6. On the last image, I used hero power again, +2 attack -1hp, +1 attack (Warbot's card text), the very last characteristics is 6-5.
P.P.S. I lost that turn and therefore that adventure run (but I suppose I would lose it anyway without that very useful +2 attack: look at enemies' hp) D: