PICO-8 0.2.3:

1. REBOOT PICO-8 to clear memory to defaults.
2. Open the music editor.
3. Navigate to pattern 63, then select it.
4. Activate the fourth channel. Set its sfx to 63.
5. SAVE NONEMPTY.P8
6. REBOOT
7. LOAD NONEMPTY.P8
8. Navigate to pattern 63.

Expected: Pattern 63's fourth channel should still be active and set to 63.
Actual: It is empty.

Expected: The .p8 file contains a music section with 63 default patterns and one non-default pattern.
Actual: There is no music section.

This appears to be a bug in the P8 section truncation logic that omits trailing default (empty) lines of each section. This does not happen when saving as .p8.png. I can set earlier channel patterns and see them show up in the .p8 while the last one is still missing. At some point my messing around restores intended behavior, so I don't know how that works internally. But I can repro with the steps above.