Strange sharp shadow edge on objects

Hello,

I am working on a game where players builds from blocks so this structure is made of 4 blocks per floor, and only on one edge of these blocks has this strange sharp shadow edge, where a block nearer to a light emitter is brighter and farther is darker. And as i uploaded the second screenshot it shows that from the other side it is okay, so only this edge on such cylinder-like blocks appears always on this side of light. These blocks are instantiated and the same. Do you have any idea, how to fix it or what is source of this bug? Thank you for your help. if you need any more specified info, i can upload, but now i am not sure now which could help you ... P.S first picture is with that bug (in the middle) and second is other side which works correctly.