The light level overlay seems to render below snow layers on nearby blocks, making it unusable in snowy environments. It does however render the numbers on blocks further away...

There are several options in the Generic category for controlling the light level overlay behavior (search for light). I believe what you want in this case is lightLevelAutoHeight. The default options are set for fastest/least checks i.e. best performance when the overlay updates.

Ah, thank you! That's the option I was searching for.