Thwomps sometimes disappear from the screen, only to reappear later. I'm not sure what causes this behaviour, but they don't seem to be invisible, they seem to temporarily cease to exist. No idea why, but it does happen with some amount of regularity.
I'm pretty sure it's related to the system that handles turning thomps off when you reach a camp, because I can get them to disappear by getting really close to the jump seed to camp, and then cause them to reappear if I back away from the jump seed. It just also happens sometimes in the middle of a level, and I'm not sure why.
I remembered to screen shot it this time.
I have a Thwomp!
Then I move up.
Where did Thwompy go? 😞
Oh, don't worry, Thwomp will come back if you move back down again. But wait, when I move up, he's gone again! Crazy Thwompy! 😂