It is not a bug. The grappling hook has a range to it symolized by the vibrating motion. The range is determined by the your distance to the hook which is why you sometimes fly far and sometimes you don't.
The game is nice and fun for in between playing sessions. Only gripe i have is that sometimes it seems to be possible to have too much space in between the enemies and the target which can cause you to die without being to do anything against it.
The game is interesting but the flame is to large if the hitbox is only the cursor, i would have probably made the flame the hitbox and left more space between the raindrops. Also the game restarting is quite off putting.
I don't know if i am missing something but there were no walls on my screen. There is also a bug that the highscore name will be replaced by intrer after completing a game.