Received response from manufacturer. Paraphrasing a bit: "Are broken and out of warranty, buy new replacement with discount%". But they we're a lot more polite which was nice.
Before they did focus heavily on wifi / connection settings, resetting sensor to factory default and testing it via cloud. There did not have any corrective effect in my case, but maybe those are prime factors for troubleshooting sensor problems like these.
Now here, we maybe need to factor in warranty periods importance when we make decisions about the future, as there was really no good solution offered that would have resolved the main problem and returned trust to the humidity values. Fragility of the sensors surprised us.
When comparing to new Shelly H&T plus-version, it seems to have even narrower operating environment than was mentioned on the original H&T versions specifications.
...still searching best solution...