I did recreate the 8-4 death bug while testing on real hardware and will resolve it soon.
I actually originally did skip the SMB1/LL level previews while building the first version, but it caused some weird level loading issues that I didn't feel like chasing down so I just restored it. I actually kind of like the level previews and the only reason they're not in SMB2U is because it's part of the character select screen, but I guess if it annoys people I could work to remove it or hide it.