True tone on external display only

When true tone is enabled and the external monitor is connected (laptop open), true tone only gets applied to the external display. (When true tone is disabled, it's the external display that adjusts.) When the macbook is open on its own -- no external monitor -- true tone works on the builtin display just fine.

I have a 2020 MacBook Pro (M1, Big Sur 11.5.1) with an LG Ultrafine 5k monitor.

Any ideas about this?