Edit: This is response to donepe's 2-year old original post, but maybe it gives some ideas for DesolationBlvd's issue.
Now that I have an account, let me chime in. Background: I've built a poly that does what the JP8 does, and a slight bit more, but when I set out to do that, I estimated the size of the effort and made it a company project, which means it's (hopefully) going to be a small scale commercial boutique run, outside of DIY scope, which is why I won't bother you with it here. But I am very
familar with the signal chain
There are three sources which can throw VCO1 off tune, aside from the CV: X-MOD and VCO MOD on the voice, and VCO1 MOD from external LM-12. I would try to selectively disable those to see if they might have an influence.
X-MOD can be suppressed by cutting off control current to IC18A (assuming IC18A itself is good). Either take out TR13A, or connect pin 1 of IC18A to V-. At your own risk, of course, but it should be safe, because for the BA662, the control current input at pin 1 is one diode drop above V-, with low resistance, anyway. Do NOT connect it to ground -> immediate classic OTA fry.
VCO MOD can easily be suppressed by grounding pin 5 of IC17 (or the same signal at R100A). Again at your own risk
, but because pin 1 is fed through 10k, the 4053 should survive the worst case 1.5mA for the test.
VCI1 MOD at LM12 should not be a source of problems, because it would affect other voices as well.
Oh, and one more thing: The synth prototype of my own design also has tuning issues, but affecting all 4 voices on a layer. This goes away when the "module controller" (in jp parlance) power connector gets contact sprayed and wiggled, or when I clip a separate analog ground connection over this power input. I have to replace the connectors with good gold plated ones, but I took note that the JP oscillators are very susceptible to the slightest ground resistance. Your issue is likely unrelated, because of the precise single location where it happens, but it should be kept in mind as a theoretical source of trouble.