I wondered if the resolution thing was the issue, it was build for a 16:9 ratio but I didn't manage to test for other resolutions (mostly because of time)
As for the health thing, I see where you're coming from, but I'd actually rather go the other direction, I think the game in it's current state is too lenient and I'd rather reduce checkpoints. The intention of not giving health back at the start of each level was to make getting through the game with no deaths harder, but since I didn't prioritise setting up other checkpoint/save systems I just decided this level based restart was the easier and more forgiving solution (I had even thought of kicking the player back to the menu but felt that was too harsh)
To be fair, you're not passing checkpoints on the way through. There's no indicator that you're getting a checkpoint at the start of each level, and the player character reflects current health in a very apparent way (short of having a health bar), I feel it is a little unfair to say that it was unexpected or a bug
We disagree about that feature, but nevertheless I still appreciate all the feedback you've given and that will help greatly for future versions of the game with bug fixes and extra polish. Thanks again Igor :)