Phone battery {Pbl} not updating on watch automatically

I have a circular progress bar for which I want to use the {pbl} tag as a shader. This only works when I first send the face to the watch (Samsung Galaxy Watch 6 Classic) - the correct phone battery value appears in the shader. However, the {pbl} tag on my watch face does not seem to update from there - it remains at the value it was when first sent to the watch. This is true for both the progress bar shader AND as a singular text tag.

Set the battery optimisation on the Watchmaker app on your Phone to unrestricted, in the App settings.