Thanks, @Zalde – you’re right. I fell prey to the “gotcha” that once you set a local property (accidentally) in global scope, that property then appears to be a global property and can no longer be set locally. (Which I also mentioned here.)
Which leads me to another feature request: It would be nice if the UI, probably the bottom toolbar, had a toggle for the “set local properties option” (maybe like the concert/transposed pitch toggle), so that this info is visible and changeable even when the Properties panel is closed. The issue is that if I switch to Engrave mode and adjust something visually with the graphic editing tool selected, there is no indication of whether I’m setting things locally or globally. This is what happened to me above – I shortened the 2nd ending line with my mouse, then noticed that it affected all layouts, then checked the Properties panel and saw that the property was global (because I had accidentally changed it globally, not because it’s inherently global).
So my workarounds for this collision issue do work, but it would be nice if they weren’t needed.