Maybe I did not get the memo, but I think Industrial Age units are not out yet. In addition, how is someone able to have 300 troop capacity (12 tanks * 25 units)?
That happens because he had 12 Crusaders and then he got auto upgraded by mistake and they turned into tanks, that take 10 troop spots each instead of 6. it just happened to me today also, not sure what I can do about it.
And if what you say is true and if this player plays fair, he would just deploy all the tanks and end the battle to get rid of bug but instead he exploits it and destroys all defenses first so he can use the tanks again on other players before ending the battle.
The bug you have is different from this one. Basically your heavy cavalry got upgraded to industrial age, but the 12 tanks in this picture is made from factory that needs 25 units space each and 3 times stronger.