OH WAIT. How did it take me literally all this time to figure it out. Garmadon has that big ol hole in his chest because he's quite literally heartless. Like the hole is even on his left side, where hearts usually go. Harumi brought back a version of Garmadon without any love. He very explicitly told Misako that he has no heart in s10, which I thought he meant metaphorically, but no this man physically has no heart in his chest.

Oh my god this was so painfully obvious, why am I only just now piecing it together.