Kirk,
just a little more information, I just discovered this tidbit (trying to find a setting which prevents the "flashing" of thumbnails and preview upon resume).
In my preferences, I had "Only allow arrangements when in Arrangement sort mode" set. I've toggled this setting off, and now it is possible to reorder the images in a Collection, and the order stays persistent after closing and reopening the Collection. Haven't tried closing/reopening PM+ yet.
Maybe this help in the debugging.
/Michael