Big shadows appear in the overworld without any blocks that block sky light

Today I discovered this big shadow while exploring the overworld. (see screenshot)

The issue is basically the opposite of MC-18526.

Things to mention:

• This happened in a buffet (plains) world
• There aren't any blocks blocking sky light
• Mobs can spawn in the shadow and undead mobs won't catch fire
• The issue affects 4 whole chunks
• It affects both client light and server light
• The shadow disappears when you reload the world