I'm happy you enjoyed it! Key mapping, including a left-handed preset, is vital for a good game, in my opinion. If I had more time, I would've definitely added it.
I don't have an excuse for the the boss hiding, haha. I knew about it, and it is something I could've fixed in 30 seconds, but kept delaying it till the last day, because there were more serious things I wanted to iron out. I focused on other mechanics, and forgot. Bosses have different movement states. One of them is "retreat", where it moves the opposite direction it faces. In rare cases, if you are jumping/dashing directly above it, it can partly retreat into the ground.