First of all I would like to apologize in advance for asking this question; I'm pretty much new to the magical realm of ROM hacking.

Anyways, my problem is that the floor in the editor appears different than in-game. This is not exclusive to this level. Included are two different screenshots (please excuse the fortnite ad) showing what my problem is, a screenshot of the level editor, and a screenshot of the Map16 data, using Helios' NSMB2 tileset.






Another question: Is the front background layer corrupting because too many unique tiles are present?



Any help from somebody who knows what they're doing is much appreciated; if any other screenshots are needed then please don't hesitate to ask. Thanks for reading.

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That looks like randomisation to me.
Some tiles get completely randomised when they are placed at specific points in map16 (tileset editor).

I recommend you to move all the tiles you see here to a different location in map16 and then try again, the randomisation should be gone.
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when the new background is made, the tiles have random orders. but, for this specific bg, some of the tiles are the flag tiles (which go to the peach castle of the original bg). those eight flag tiles get rewritten with a different thing, but the animation still plays, so the tile is just a pink flag silouette (or nothing at all).