Right. I get no banding with kernel 121 with that clip so the plot thickens.
I paused the vid and snapped it:
Then if I go
echo 8bitnow | sudo tee /sys/class/amhdmitx/amhdmitx0/attr it switches to 8 bits and I get a noticeable difference (look just above the 29 flag):
On my TV (Panasonic) that is typically the difference between 8 and 10 bits. Is this similar to what you are getting? If not, then we need to look elsewhere for the cause of banding.
You can check what bit-depth is being sent to the display with
cat /sys/class/amhdmitx/amhdmitx0/config On kernel 121 it should say
VIC: 93 3840x2160p24hz
Colour depth: 10-bit
Colour range: limited
YCC colour range: limited
PLL clock: 0xc000027b, Vid clock div 0x000a339c
audio config: off
3D config: off
On kernel 119 it should say 10-bit YUV444 and the line PLL clock will be different.
If you are playing around with bitdepths and get blank screen,
echo now | sudo tee /sys/class/amhdmitx/amhdmitx0/attr will usually fix it.