The only “issue” that I could spot is that inotify-tools is specified as optional dependency.
$ pacman -Si grub-btrfs
Repository : community
Name : grub-btrfs
Version : 4.12-2
Description : Include btrfs snapshots in GRUB boot options
Architecture : any
URL : https://github.com/Antynea/grub-btrfs
Licenses : GPL3
Groups : None
Provides : None
Depends On : btrfs-progs grub
Optional Deps : snapper: For snapper support
inotify-tools: For grub-btrfsd daemon
Since starting grub-btrfsd fails because of it not being installed, I think it should be specified as grub-btrfs’ dependency.
Perhaps you might want to report it to the developer?