Why are these staves overlapping?

A mystery. No frame breaks anywhere. Here are some screenshots:

I can add a frame break to force it to spill to the second page (and I will ), but I’d still like to know why this is happening.

It’s not a global thing, because other layouts with tab in the same project look fine. Oh, and Ideal Gap for tab staff is set to 4, the default.

Any ideas? I can’t post the file here, but I can share it via PM.

Is casting off fixed to x systems per frame, in Layout Options?