"underground" is badly defined.

[keybounce]

Transparent blocks and outdoors

Any block that permits light to travel through (including torches, fenceposts, and glass) will permit instability affects to pass.

This is probably not intended. A switch to testing for "can lightning strike here?" or "do any of the blocks above me have a 'you cannot walk here' size of a full block?" would probably be an improvement.