The hplip package prior to version 3.20.3-2 was missing the compiled python modules. This has been fixed in 3.20.3-2, so the upgrade will need to overwrite the untracked pyc files that were created. If you get errors such as these
hplip: /usr/share/hplip/base/__pycache__/__init__.cpython-38.pyc exists in filesystem hplip: /usr/share/hplip/base/__pycache__/avahi.cpython-38.pyc exists in filesystem hplip: /usr/share/hplip/base/__pycache__/codes.cpython-38.pyc exists in filesystem ...many more...
when updating, use
pacman -Suy --overwrite /usr/share/hplip/*
to perform the upgrade.