Tfw the download of the episode takes longer than finding the issue.
It's a bug in level loading. The level extension isn't automatically searched for if none is provided.
Add .lvl to the file loaded by the warp to circumvent it. Right here (this is in 1-2 for example):
You could probably automate the process with a couple clever regexes (by which I mean, you totally can. That's why this post is so long). Here's what I came up with for Visual Studio Code. First, open the folder. Then, go to the "search" tab.
1) Numeric level files.
2) Level files that have letters and then numbers:
Interestingly, most of these already suffix the .lvl. There just appears to be one instance where that's not the case, so that one can simply be handled manually:
That ought to take care of things. Red's looking into fixing it generally for SMBX2.