Apple put ProMotion in the built in display, so while it can ramp up to 120Hz, it'll idle at more like 24 Hz when showing static content. (the iPad Pro goes all the way down to 10Hz, but some early sources seem to say 24Hz for these MacBook Pros.) There may also be panel self refresh involved, in which case a static image won't even need that much. I bet the display coprocessors will expose the adaptive refresh functionality over the external display connectors as well.