Flex displays Netflix Dolby Vision in 720p format

Here's the deal: I have an LG C9 (2019 manufacture date, software version 05.00.03). It's connected via HDMI 3 to an Xfinity Flex box.

I can say with certainty that when I play a Netflix Dolby Vision video the display mode is 720p

My network connection is good. The network test indicates about 89Mbps.

When I access Netflix using the Netflix app installed on the TV the display mode  is 2160.

Why am I only getting a 720p display resolution when I access Netflix using the Flex box? It should be 2160.