OH!
I'm surprised to hear you haven't played Castlevania, I assumed that's where you got the stairs from, aaaaaaand huh now that I think about it that's probably why I thought they had collision issues! In Castlevania you're holding up to maintain movement up the stairs, pressing up again just moves you further up the stairs (since jump is a button). Since your game tied the up button to jumping, I probably pressed it without remembering it was a jump, assuming I had to hold down up the whole time I was on them... especially because when you "jump" off the stairs in your game currently you don't really gain height, you just fall straight through.
What kind of one button platformers did you play if I may ask? I'm curious to try some, even arcade games like Donkey Kong, Mappy or Bubble Bobble have a dedicated jump button so this perspective is interesting to me