Nothing in the lower difficulties sections would compromise the file to the point that it’s not properly read in game. It’s something different in the new build, but not the notes that actually pass Magma scrutiny.

I used to run into this. I don’t remember exactly what I discovered that causes it, but now every time I do a new build of a song, I name it something different. This forces Rock Band to see it as a “new track” and re-loads it, without having to erase your cache (which is never necessary in my experience.)

For example:

Build 1: josie_rb3con

Build 2: josie_v2_rb3con

So on, so forth.

I know for sure that I’ve run into the issue you mentioned when reverting to an old file name that the cache recognized, with a new build. So if I went from v1, to v2, then did a new build as “v1” again, this would happen.

My guess is the game sees the track and says “yeah I recognize this, no reason to load it again (because it takes longer that way)” and then when it attempts to load the data from the cached file, that particular file no longer exists (because it’s been rebuilt as something else). That is a complete guess, though.

Last note: I’ve never had to change the Song ID when doing this. The only time I change a Song ID is if I notice that it has the same ID as another song in my library, which will prevent it from populating in your song list in game (even though the game still shows it as “loading a new song” when it’s building the library on boot.)