I’ve recently switched over to Manjaro too and have the exact same problem.
Everything works fine in this beautiful OS except after sleep, desktop effects don’t work.
Pressing alt+shift+f12 makes them work again, but this isn’t a permanent solution.
By the way, mine occurs on a fresh install too, so I believe this is a bug.
I’ve currently installed the latest AMDGPU drivers and everything works as expected. All my programs are utilising the driver properly, sleeping and waking works fine, gaming works, video editing etc.
I only have one 1080P monitor connected to my RX590 over HDMI.
I also believe this is Manjaro specific as I didn’t get this behaviour when I was on a debian based system previously.
Anyone have any further ideas?