Apperently Dry Bowser gives up before Mario even starts to move. I finally finished making the airship level in the first world 8 castle. I replaced the boss arena with the original so that I knew it would work. Well, sort of anyway. When Mario reaches the bridge and Bowser jr does his dance, Dry Bowser falls out of the sky in pieces and is already defeated. Bowser Jr jumps out of the cutscene and locks mario in. Mario is still standing there. Nothing. Softlock. Why does this happen?
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NSMBUDS - Discord Server 1 - Discord Server 2 - Youtube Channel

Not sure if I got this right. But if you changed the boss to another boss that usually isnt in that level, the game loads wrong stuff, leading to a softlock.
Apply this ASM hack and you should be fine. This ASM hacks makes the game load the right boss files.
If my assumption is correct, this should fix this problem.
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previously (un)known as BooChewbaca

Nope, the boss is the Dry Bones bowser, which I am working on in the level 8-castle. I never touched the boss area, I only made the level look like an airship. I'll try to post a video to show you what actually happens.
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NSMBUDS - Discord Server 1 - Discord Server 2 - Youtube Channel