Zwift Trainer Difficulty setting not working on Pixel

To expand on what @Bob_Gorman_TFC hinted at above:
Trainer difficultly slider has an effect on resistance only in free ride mode. It’s job is to tweak how steep hill climbs and hill descents feel (if you have a resistance-controllable trainer).

It has no effect on resistance in ERG mode (i.e. when you’re in a Workout and your avatar has a glowing panel in front of the handlebars). Some more information on ERG mode in our Support Hub

Lastly - since you’re using a Pixel: Android devices support controllable trainers if you’re paired over Bluetooth. Support for controllable trainers over ANT+ is not yet supported (real soon, though).

EDIT: As of December 2020 game update, there is support for controllable trainers over the ANT+ FE-C protocol. Please see this thread