More frustrating test results:
I've edited it as follows:
Priority 1 civs are the East Asians the New World civs. East Asians like silk and New Worlders like dye.
Priority 2 civs are the 'tropical' civs (Sub-saharan Africans and India), who like jungle and don't tundra. regionSize = 6
Priority 3 civs are everyone else, haviig various terrain preferences which I won't bother going into. regionSize = 6
I've been testing on continents and in every test I've done the priority 1 civs always starting near their desired luxuries, resulting in successful groupings of New World and East Asian civs.
Unfortunately the priority 2 civs, the tropicals, often don't start anywhere near jungle, even though priority 3 civs do!
So it seems that priority 1 civs do get priority, but priority 2 civs are not getting priority over priority 3 civs :/
I've attached my latest file