The Vintage Garden Book of Roses stated that they, the nursery proprietors, had had reports of rebloom on Alba maxima. Alba semiplena, maxima and suaveolens are all members of the same sport family. I don't have a citation, but I believe it has now been determined that the albas came from a (probably) spontaneous cross between R. canina and a Damask rose. We do know that some of the Damasks do rebloom, so I think it is entirely possible that the albas may have inherited a reblooming recessive gene which occasionally expresses as scattered fall bloom.
This is so interesting ! I just read hypothesis about the origins of Albas, I think there is still a doubt about their lineage, and also I was wondering how they behaved in different climates. Another bloom that surprised me the same day was from rosaceae Cydonia Oblonga.
There is an article in the newsletter of the World Federation Of Rose Societies ("By Any Other Name"issue 26) in which Karin Schade explains her work on the Alba group. The editorial note states that the origin of Albas is still uncertain. I will send you the link in a private message.