I've created a resource pack that is designed to darken blocks that monsters are able to spawn in. it's designed to be used with smooth lighting disabled.

The lightmap file is a 4 color image, so the threshold between blocks should be distinct. However with the latest release, there appears to be bleedover from the darkened areas into the lighter areas, which makes the threshold somewhat unclear and more importantly is discontinuous with the file.

Note that I used a 2 color lightmap for the image above. It is provided below.

fabric api and modmenu were the only other mods I had installed. both are at their latest versions.

This might be an effect of light intensity.