Looks like you need a workaround:
const device = await Homey.devices.getDevice({ id: '45f27858-c506-4d3c-a800-1954d803e3e3' });
const cap = device.makeCapabilityInstance('measure_power');
console.log(cap.value);
I think it has to do with lazy evaluation, where capability values are only updated when explicitly required.
