I have a similar issue, RX 5700 XT with one monitor connected via Displayport. If I leave the xfce display settings to not configure new devices when connected the screen will go blank after 5 minutes for a few seconds before coming back, will never go to sleep. If I enable profiles and to configure new displays it’ll go black and stay black but will never come back on, have to reboot.
journalctl shows the following messages:
Sep 09 06:40:58 Manjaro kernel: [drm:dpcd_set_source_specific_data [amdgpu]] ERROR Error in DP aux read transaction, not writing source specific data
Sep 09 06:40:58 Manjaro kernel: [drm:retrieve_link_cap [amdgpu]] ERROR retrieve_link_cap: Read dpcd data failed.