Hello btrfs users! Testing needed!
All btrfs (and grub) users here, please test the following (same commands as above) and report the results here!
Maybe we can make this a package later, if this works for all. 
cd /tmp
git clone https://gitlab.manjaro.org/packages/community/os-prober.git
cd os-prober
makepkg -cf
sudo pacman -U os-prober-1.*.pkg.tar.* # this line installs it