With [Slot Blend Modes] component, the shadow of the bird became square (wait for the eagle to appear).
Here is when Slot Blend Modes not added. It doesn't have square shape but since it is not multiply, the color is wrong when flying over some terrain.
Pre-multiply is checked in Export setting.
Note:
I found that the shadow is on the 2nd exported spritesheet. Not sure is this is related. I have some other single spritesheet scenes which also use shadow and is working fine by just adding [Slot Blend Modes] component. This one is too big and cannot fit into a single spritesheet.