I’m back to looking at this and not sure what I’m doing differently, but… I’m definitely seeing the PWM frequency shift with an AS3340A. At least if it’s powered as (or rather, like) in the datasheet, with -12 V via 680R to pin 3. If I connect a regulated -5 V to pin 3 then the frequency shift is maybe gone or maybe just very much reduced. (I also find the CV scaling changes, so if I just switch the power without adjusting the tracking trimmer I get very much different frequencies.) I need to look at this a little more carefully and maybe research some of the other remedies that have been mentioned, but Alfa recommends the regulated -5 V in any case.