I have a tree whose leaves are planes with an opacity map. I've noticed that, when activating the opacity map, this is what shadows look like.
This issue happens only with the shadows projected by the leaves. If I aim the camera at the leaves they look fine.
The only fix I found was changing the Filtering Type to closest for the opacity map.
I lack a technical background so maybe this is how it's supposed to work. My apologies in that case.
I hope this saves some time to other new users like me.
EDIT: I think it only happens with directional lights.
Thanks for reading!