I run Manjaro i3 on my desktop with both a 4K monitor connected via DisplayPort and a 1080p monitor connected via HDMI. When I suspend my machine, the DisplayPort monitor shuts off and fails to correctly turn back on when the system resumes - everything is moved to the 1080p monitor and the 4K monitor remains off.
I've so far found two workarounds for this issue, the first is simply power-cycling the monitor before resuming the system and the second uses
xrandr. Neither of these is a perfect solution, but it's better than rebooting!