Thanks Ken.
The info you've provided looks very helpful, and I'm about to start utilising it in investigating further.
It's probably just some bug in the way doubledoubles are assigned (or being read), but I think it's about time I at least worked out what is going on.

I was going to look in "lib/unicore/TestProp.pl" but I can't locate it

Comments at the start of TestProp.pl inform us that it is machine-generated by ..\lib\unicore\mktables from the Unicode database, Version 14.0.0.
I guess this would be done during the "make" (or perhaps "make test") stage.

I'll post again later - once I've done some digging.

Cheers,
Rob