Bug: NNEDI3 simply doesn't work with MT. It causes image corruption.
Code:
SetFilterMTMode("DEFAULT_MT_MODE", 3) #neither 2 nor 3 works
AviSource("Preview.avi", audio=false, pixel_type="YV12")
nnedi3_rpow2(2, Threads=1)
Prefetch(8)
So this looks like a bug in AviSynth+. Without MT, it works.