I was about to report having the same bug but it seems the today's Mesa
19.2.0-2 update has solved the problem, I will include what I was originally
going to post in case it's of use to anyone.

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I've began to have this bug in the last week or two, I have two monitors one an
Asus PA238 connected via DVI has no issues, the other a AOC 2270W connected via
DP changes to a lower resolution after wakeup, typically 800x600 which cannot
be changed back to normal.

I can reproduce this consistently by using 'xset -display :0 dpms force
standby' and waiting about one minute.
Running 'xrandr --output DisplayPort-2 --auto' restores the display to its
normal native resolution.

The system log shows the following error:
[drm] enabling link 2 failed: 15
[drm:dm_restore_drm_connector_state [amdgpu]] *ERROR* Restoring old state
failed with -22
Stack trace: https://pastebin.com/yguN5NUk

System specification:
OS: Arch linux 5.3.1-arch1-1-ARCH
GPU: AMD Radeon RX 590 using AMDGPU-Pro
DE: KDE Plasma 5.16.5