Due to an upstream change, the AMD microcode now needs to be loaded via the boot loader, in the same way as the Intel microcode is loaded.
[color=#121112][size=small]All users of AMD-APUs/CPUs should install this update using the following commands.[/size][/color]
sudo pacman -Syyu
sudo pacman -S amd-ucode
sudo update-grub
Here is the upstream update announcement:
https://forum.manjaro.org/t/stable-update-2018-08-31-kernels-mesa-qemu-nvidia-firefox-python-haskell/56864