I'm sure it's been solved already, but I had luck using:
.setLightLevel(s -> 15);
instead of:
.setLightLevel(0.5f);
on 1.16.5.
(replace numbers with the value you want)
Just add this to your block register class with the other properties.
This worked for me on 1.16.5 using Forge MCP mappings. It will likely be different on Mojang official mappings. Hopefully this helps somebody out there who might come across this thread.