Adding modprobe.blacklist=vc4,v3d to the kernel parameters, and the current rpi firmware, mainline kernel and vc4-kms/fkms-v3d-pi4 overlay, fbturbo will create a 24/32 framebuffer. With the vc4 module loaded, fbturbo will only create a 16/16 framebuffer.
Plasma displays properly now, problem solved.