maybe not related to mvtools, but the following shows errors introduced in the "float32-int16-float32" process will actually affect the final result even if it's just 8bits
Code:
import vapoursynth as vs
core = vs.get_core()
clp = whatever
clp = core.std.ShufflePlanes(clp, planes=0, colorfamily=vs.GRAY)
clp = core.fmtc.bitdepth(clp, fulls=False, fulld=True, bits=32, flt=True, dmode=1)
clp = core.fmtc.transfer(clp, transs="470bg", transd="linear", fulls=True, fulld=True)
clp1 = core.fmtc.bitdepth(clp, fulls=True, fulld=True, bits=16, flt=False, dmode=1).fmtc.transfer(transs="linear", transd="srgb", fulls=True, fulld=True).fmtc.bitdepth(fulls=True, fulld=True, bits=8, flt=False, dmode=3)
clp2 = core.fmtc.transfer(clp, transs="linear", transd="srgb", fulls=True, fulld=True).fmtc.bitdepth(fulls=True, fulld=True, bits=8, flt=False, dmode=3)
dif = core.std.MakeDiff(clp1,clp2).std.Expr ("x 128 - 100 * 128 +")
dif.set_output ()