[age]

you have used different transfer values,probably you have to go in linear space using 709 transfer
or come back from linear space using "61966-2-1"

orig = c
c = core.fmtc.resample(c, w = 1920, h = 1080, css = '444', kernel = 'spline36')
c = core.fmtc.matrix(c, fulls = False, fulld = True, mats = '709', col_fam = vs.RGB)
c = core.fmtc.transfer(c, fulls = True, fulld = True, transs = '709', transd = 'linear')
#c = core.fmtc.transfer(c, fulls = True, fulld = True, transs = '61966-2-1', transd = 'linear')
c = core.fmtc.primaries(c, prims = '709', primd = '2020')
c = core.fmtc.transfer(c, fulls = True, fulld = True, transs = 'linear', transd = '2084', cont = 1000.0 / 10000.0)
c = core.fmtc.matrix(c, fulls = True, fulld = False, matd = '2020', col_fam = vs.YUV)
c = core.fmtc.resample(c, css = '420')
c = core.fmtc.bitdepth(c, fulls = False, fulld = False, bits = 10, dmode = 5)

z = c
z = core.fmtc.resample(z, css = '444', kernel = 'spline36')
z = core.fmtc.matrix(z, fulls = False, fulld = True, mats = '2020', col_fam = vs.RGB)
z = core.fmtc.transfer(z, fulls = True, fulld = True, transs = '2084', transd = 'linear', cont = 10)
z = core.fmtc.primaries(z, prims = '2020', primd = '709')
z = core.fmtc.transfer(z, fulls = True, fulld = True, transs = 'linear', transd = '709')
#z = core.fmtc.transfer(z, fulls = True, fulld = True, transs = 'linear', transd = '61966-2-1')
z = core.fmtc.matrix(z, fulls = True, fulld = False, matd = '709', col_fam = vs.YUV)
z = core.fmtc.resample(z, css = '420', kernel = 'spline36')
z = core.fmtc.bitdepth(z, fulls = False, fulld = False, bits = 8, dmode = 5, ampo = 1, ampn = 1)