I often face this issue. The fingeprint sensor does not work and sometimes the touch also misbehaves when phone is charging.
Why does this happen and how to fix this?
I experienced this issue with Mi Note 4, as well as Redmi 4A.
Something I have noticed with my fingerprint sensor is that if phone is face down and hasn't been used for a short while, phone will not read finger print until phone is actually picked up. Not sure if because of proximity or orientation sensor, but appears to be that way to keep from accidental unlocks. Is your phone face down when attempting to unlock? Also, what do you mean by "touch misbehaves"? If you mean the entire touchscreen and then combined with fingerprint sensor this could be pointing to your device being defective.