The capability to sync mapped LAN drives on a Windows PC was in fact available in the very first versions of the Windows version of Proton Drive. The sync function stopped working after an update shortly after launch, and has never been seen since then :-(
When I back then contacted Support about it, they didn't recognise the fact, that it actually had been a working functionality, and just stated that this is not supported.

As mentionned here https://protonmail.uservoice.com/forums/932839-proton-drive/suggestions/50477640-qnap-nas-integration
I am also interested in the ability to backup Proton Drive to my NAS.