Same NAS, same file recalbox-boot.conf on Stable Recalbox and Beta Recalbox, and beta doesnt detect Shared folder. On Stable recalbox works fine. Some must could be changed on beta version.
EDIT: I solved it adding ,vers=1.0
Try sharenetwork_smb1=ROMS@192.168.178.1:recalbox/roms:username=recalbox,password=Recalbox,vers=1.0