Massive screen corruption with the modesetting driver (works with amdgpu)

My monitors:

• Ancient 1440x900 monitor (PLE1900WS DP-1-5) connected with VGA to iGPU (Intel UHD Graphics 630)
• Less ancient 1600x900 monitor (Samsung SyncMaster HDMI-1-3) connected with DVI-D to iGPU (Intel UHD Graphics 630)
• Primary: 1920x1080 monitor (PL2288H DVI-D-0) connected with DVI-D to dGPU (AMD RX 590)
• Slightly ancient Sony TV 1360x768 (HDMI-A-0) connected with HDMI to dGPU (AMD RX 590)

Using the modesetting driver I get this on DP-1-5:

While it works just fine with the amdgpu driver (I have a different issue with the amdgpu driver, which is why I was trying the modesetting driver instead: xorg/driver/xf86-video-amdgpu#11 )

• This happens only if the monitor is set to certain resolutions; it happens with 1440x900 (highest) but not with 1280x1024 or 1290x960.
• It happens both with PRIME Sync on and off.
• It does not occur if the Intel card is set as the default GPU with xrandr --setprovideroutputsource.
• Nor does it occur if DP-1-5 is connected with a (passive) HDMI-->VGA adapter to the dGPU instead.
• The cursor is not effected by the screen corruption, everything else is though.
• This happens both after log-in into KDE Plasma, and before log-in in SDDM.

This worked fine before I switched out my Nvidia card for the AMD card. Prime Sync was enabled, Nvidia was set as the renderer. Monitor configuration was identical. Intel card was using the modesetting driver.

Here's my Xorg log: Xorg.0.log

• linux-5.6.7-gentoo
• mesa-20.0.5
• xorg-server-1.20.8
• xorg-drivers-1.20-r2

@daenzer over at amdgpu suggested that it might be a pitch and/or tiling parameter mismatch

Please let me know if any more information is needed.