As NVIDIA already stated, nvidia-open is not supporting older cards at all.
mhwd adopted nvidia-driver-assistant which is used to define if the hardware is supported by open or proprietary drivers. Maybe that tool suggests the wrong driver on your device. These are only for Turing and newer architectures …
Sources can be found here: Packages / extra / nvidia-driver-assistant · GitLab
Example to check if the open driver is used from mhwd-config: [[ "$(nvidia-recommended-driver)" == "open" ]] && DEPKMOD="nvidia-570xx-open"