OK. This is solved. First cup of coffee morning revelation.
For each part there are a set of what I'll call "fixed" properties, including Name, ID, Partition, Symbol Name, Name Inverted and *lo and behold* "Forward to PCB". Selection choices for the "Forward to PCB" property are Inherit from Definition, True, False. I never noticed this until this morning.....
I deleted the manually added User Property, applied this "fixed" property to the BOM only items and it all now behaves as expected.
I also note that Borders have a property FORWARD_PCB with a value of "0", which also does not show up in the Property Definition Editor.
Why don't properties like these show up in the Property Definition Editor list?
Why are these properties inconsistently handled?
Ken