Yes, I know. Optifine is evil.
Currently, the light output does not work with optifine.
I think it is a wontfix.
I'll still see if I can find something about it and if so I'll try to fix it myself. But it's unlikely, as I have no idea about either optifine or forge.
But maybe I'll find something anyway.

Strange, I guess optifine completely ignores and replaces Minecraft's native lighting updates(?). I wonder if placing and removing a torch next to it would cause an update.
If you can find something, that would be amazing. Otherwise, I'll look into it as a low priority issue. Optifine support for lighting would be nice but not critical.

@dannydjdk yes it cause an update. (edit: or not?)
is just not quite as critical.

Optifine is ignoring the WorldLightManager's checkBlock() method.
I'll try not to lose any sleep over this one, but if a workaround surfaces, I'll try it.

@dannydjdk sleep is important!
Many mod developers do not support optifine.
It is closed sourced and therefore very difficult to support.
Yes it would be great.
But it also works without.

Just as a question to myself (so as a note):

Is it possible to use blockstates to display the current light intensity?

We are overriding the Block's getLightValue(BlockState state, IBlockReader world, BlockPos pos) which is what vanilla Minecraft uses to get the light level for the BlockState. It seems as if Optifine is honoring that. It just isn't honoring the call to the WorldLightManager's checkBlock() method. So, it doesn't update the lighting when it's supposed to, but when it does decide to update the lighting, it seems that it's doing that correctly. Right?