I can also confirm this
I do/did the same as you I only have PUPs checked for on-demand, I have just checked and PUPs are enabled, so at some point I have enabled Hardened Mode.
I was thinking it is an unfortunate consequence of Hardened Mode checking all files against the avast cloud, but that is only meant to be for, Hardened Mode, aggressive.
I have also found that having enabled Hardened Mode, on-access scan for PUPs are enabled. Yet if you disable Hardened Mode, the on-access scan for PUPs isn't reversed.