thank you for playing!
i was terribly scared we wouldn't be able to get all the levels done in time, so they were a bit rushed design-wise. originally, they were supposed to be designed with a lot of verticality, hence the movement.
as for the damage, there is no state or condition in which the player can't take damage, so I'm not sure why they don't. as well, the explosion works very similarly to the bullets, so i couldn't figure that out. one idea I've considered, but not looked into, is the physics engine being buggy. although the explosion and the bullet deliver damage in the same way, they do have different systems of detecting collision. explosion checks using an area, whereas the bullet is obviously a ray-cast, though I'm not entirely sure if that is the reason why.
what would be useful is to know your computer specs, so I could tell if this is a common problem based on that. if you could, could you tell me your OS, CPU, and any other information you may think is important or relevant? just OS and CPU should be helpful enough, but yeah anything else would be nice.
anyways, sorry you were unfortunately one of those with the damage bug, super glad you enjoyed the game nonetheless though!
and also, thank you for the lead on the bug! I'm looking into the explosion code.