SUMMARY
***
Somewhere around 5.27 PRIME got completely broken for my configuration (see below), and started causing graphical errors when enabled. Rolling back to a build in january fixes the issues. 5.27 caused the system to hang and become completely unusable. 5.27.2 makes the system usable in this configuration, but with graphical errors. (Mostly limited to taskbar, launcher, etc).

The dummy plug (or a connected display) is required to enable the iGPU for vaapi, but having 1 iGPU display and 1 dGPU display will also produce the same result.
***


STEPS TO REPRODUCE
1. set DRI_PRIME in /etc/environment
2. plug a display in to iGPU and dGPU
3. 

OBSERVED RESULT
Rendering/gfx errors, pink or garbled taskbar, launcher, etc

EXPECTED RESULT
No rendering errors

SOFTWARE/OS VERSIONS
Operating System: EndeavourOS  (Arch-Based)
KDE Plasma Version: 5.27.2
KDE Frameworks Version: 5.103.0
Qt Version: 5.15.8
Kernel Version: 6.2.2-zen1-1-zen (64-bit)
Graphics Platform: Wayland
Processors: 20 × 12th Gen Intel® Core™ i7-12700K
Memory: 30.7 GiB of RAM
Graphics Processor: AMD Radeon RX 5700 XT
Manufacturer: ASUS

ADDITIONAL INFORMATION
Mesa 22.3.6
lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation AlderLake-S GT1 (rev 0c)
03:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Navi 10 [Radeon RX 5600 OEM/5600 XT / 5700/5700 XT] (rev c1)

2XDP Displays connected to 5700xt
1XDP Dummy Plug connected to iGPU

/etc/environment
#
# This file is parsed by pam_env module
#
# Syntax: simple "KEY=VAL" pairs on separate lines
#
#QT_QPA_PLATFORMTHEME=qt5ct
#QT_STYLE_OVERRIDE=kvantum
BROWSER=firefox
EDITOR=nano
GTK_THEME=Orchis-Dark
DRI_PRIME=1
LIBVA_DRIVER_NAME=iHD (switching to i965 has no effect)