Thanks. I'll look into that.

But the fact that that world is different means that at least some of the ROM is different or in a different order. One video I watched claims that in the NES version, the address checked when attempting to warp to world 36 is in the enemy data address table. So presumably then, either the enemy data address table is different in the FDS version (because the enemy data is in a different place or the FDS addressing works differently) or it is in a different place in the ROM. So that's at least one difference.

I would just run a diff, but I can't tell which ROM files online come from which game, since most are labelled US/Japan.

Is there a complete list out there of differences between the SMB1 ROM files from the FDS release in Japan and the NES release in the U.S.? Normal gameplay is apparently identical, but there are clearly differences in the ROMs (e.g. in world 36). I haven't had any luck looking on Google.