Plums keep ripening when it gets cooler, there are enough warmer days in between the cooler ones that they ripen at a slower pace. Heck apples keep ripening until mid-late November for me.
@WildApple23 it is not too hard to control black knot on a small tree, just cut off smaller knotted limbs and blowtorch the knots on larger limbs as soon as you see the knots. I only lost one tree to knot and it was hyper-susceptible so I’m glad I got rid of it. In any given year I remove maybe 50 knots from my 20 or so plum trees. Occasionally there is a really light or really heavy year.