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When you use either a Short Sword or a Sickle in one hand, and you block with a heavy shield in the other, the animation bugs out. Eivor doesn't plant the shield into the ground.
I can't test this as I don't have access to it, but I imagine that this bug could also be happening with the Scythe weapon type which is currently hidden in the game files and that is likely going to be added with Siege of Paris.
0:00-0:20 Short Sword, 0:20-0:40 Sickle, 0:40-1:00 Bearded Axe (Demonstrating how the animation is supposed to look like)