Not sure if this is a "feature"/something I need to configure or a bug. I mined some coal ore (using ~ left-click) and after the blocks were removed it shows up black where they should be. I added a shader after that to see if that fixed it, but still there.

Image is without shader: https://i.ibb.co/jzvVQp0/2020-05-20-14-48-17.png

Image with shader (BSL): https://i.ibb.co/QCHh4SF/2020-05-20-14-58-47.png

This is coal gas... somewhat deadly

If you enter coal gas your world may fog
Swim underwater to wash out your eyes

Actually, it looks like the coal gas effect stays but the fog clears, right?

Ironic part here.. when I rejoined the little help message on loading screen mentioned this. How long do I need to swim? Stayed under until I ran out of breath and it's still there.

Right... the fog effect on the world (for walking into the gas) is removed by swimming... the gas stays... it can dissipate if enough open blocks/light level. Can also place blocks such a dirt to replace/remove gas.