This issue is resolved now - incompatible item selector, I think.
The grav-skeleton-bootstrap4-site has Children instead of @self.children in the options. Maybe this is shorthand but I went back to my website and got it working.
To repeat a note from my other thread, using taxonomy to select pages can be confusing before you have worked out how to do it!
Sometimes my selectors in the front matter and the options tab were out of step. When there is a problem I start with one and check the other.