MDegrain with 32bit source outputs black frame

[DTL2020]

As from https://forum.doom9.org/showthread.php?p=1957326#post1957326

ConvertBits(32)

super_search = ConvertBits(8).MSuper(rfilter=4)
bv1 = super_search.MAnalyse(isb = true, delta = 1, overlap= 4)
fv1 = super_search.MAnalyse(isb = false, delta = 1, overlap= 4)
MDegrain1(MSuper(levels=1), bv1, fv1, thSAD=300, thSADC=150)

Output is black frame (in VirtualDub after ConvertBits(8)). Same with MDegrainN and with latest sources.