Interesting.
You compare actual to ESP32 above. What was being compared?

What are you using to determine actual temperature and humidity?

Interesting, it never occurred to me that he wasn't calling the functions - that would definitely explain it.

Maybe you need a newer version of Arduino core?

I find the versioning of this stuff confusing, at best.

Those calls seem to work for me. Note: there's been plenty of discussion of non-linearity of the ADC.
See:
https://github.com/espressif/esp-idf/issues/164

The big question - did tweaking these settings achieve the desired ADC behavior?

What a coincidence! That's EXACTLY the same ESP32 I use. I've got two, they work perfectly (so far?) Width: /* * Set the resolution of analogRead return values. Default is 12 bits (range from 0 to 4096). * If between 9 and 12, it will equal the set hardware resolution, else value will be shifted. * ...

I wonder if using Serial.print() in the ISR is causing the problem?

That's a fine solution. Or, you could experiment with with changes to the compiler command, probably in a makefile or config file, to ignore this error.

select() has a timeout argument which can be used to set no block, block until timeout, or block forever.