Following another report of this problem from another customer, we continued our investigation and did finally replicate an issue on their hardware. This issue is likely to share a root cause with the one you have been (patiently) reporting from your project. To quote our engineering team:
"[…] Suspicion is falling on a change in the way we use the ADC peripheral [in impOS 36] which is not documented as affecting its input impedance, but appears in practice to do so, to the detriment of users trying to feed it from a high-impedance input.
We’re looking into a fix, but have already identified a potential workaround: if you use hardware.sampler on that pin, even if you don’t take many samples, then this uses the ADC peripheral in the original manner and so should produce the “release-34” values even on release-36."