a single layer of snow does not prevent mob spawning, but the light level renderer does not render anything for them.

It does render, it just renders inside the block.

There is an option in Generic -> lightLevelAutoHeight that makes it render on top of the block's top surface.
It's not enabled by default since it changes behavior from the old defaults, and causes sort of confusing things like rendering the light level on top of basically all blocks, like all heights of snow layers, on top of slabs etc.

And to "fix" that, there is another option lightLevelCollisionCheck which causes the light level to be skipped for blocks that have a collision box that would prevent spawning. That option also has at least a minor/small(?) performance impact for updating the overlay.

I would prefer to have that as default, but great that you already implemented fixes for this edge case :D