Lighting bug

Recently while I was mining, I started to do a starcase so I could reach the surface, in doing so, I noticed this shadows appearing as if there was no light source near, though there is a torch right next to the block, this usually happens  while doing a starcase to the surface. this is the only way i could recreate the bug, but it has also appeared while beeing in a cave illuminating with torches and mining some ores. I would appreciate a lot if you could have the time to check this.

Thanks for your time.