The Copper Golems hit box is to large to fit through open single doors.
To recreate just make a 1 block wide hall way with a copper chest on one side and a standard chest on the other. place a door in the middle and open it, the copper golem will attempt to get to the copper chest but get stuck on the door,
Environment
To recreate just make a 1 block wide hall way with a copper chest on one side and a standard chest on the other. place a door in the middle and open it, the copper golem will attempt to get to the copper chest but get stuck on the door,
It's actually a path problem, its artificial intelligence doesn't allow it to cross through places where its hitbox is capable of crossing, for example 1x1.5 blocks.