I’d like to thank everyone who chimed in. I really thought that there was a link to an update involved. Turns out that the stock AIO cooler in my “ibuypower” PC just sucked (Asetek, I think), and happened to stop working as effectively at the time the update was released.
I’ve replaced it with an Arctic Freezer II, with MX-4 thermal paste, and the temps hover in the mid-50s to 60C. Managed to shatter the tempered glass side panel in the process, but hey, the CPU is cooler now.
Apologies for thinking this problem was something it wasn’t.