I have found tuning Karplus Strong to be a tricky subject. I feel one of the problems you may experience when going down the scale is the delay buffer size is suddenly increased, so your feedback loop is containing more historic data at a higher pitch. I also had a lot of problems with filters in the feedback loop affecting tuning. The original paper describing the technique shows a simple 1st order feed-forward filter, the delay time for this is constant and can be used when calculating delay times for tuning, most of the audio filters in VCV are IIR, so the setting of the filter will also change the tuning. A lot of the time when I see this used in both vcv rack and modular videos, the sound is not normally tuned to work with V/oct
The original paper can be found : https://www.academia.edu/2750161/Digital_synthesis_of_plucked-string_and_drum_timbres
I also have a module in the vcv library dedicated to tuned Karplus Strong that may give you some ideas : https://library.vcvrack.com/StudioSixPlusOne/KSDelay