They hardcode DNS servers into the TV. You need a firewall in front of the pihole intercepting that traffic, I use OpenWrt.
I nat all my dns traffic to the pihole, but looking at the logs my lg tv is using the dhcp provided ones.
Of course DOH would break that but I don't get the feeling it's bypassing it.
Given how cheap screens are, I'm puzzled as to why keeping a smart-ass TV is worth this effort?