This problem occurs in single player and all online modes.
It happens with the unmodded vanilla version of the game.
The problem happens regardless of the framerate.
The block key input is bound to -left shift.-
The game seems to struggle with recognizing the keyboard input if it happens at a certain speed.
Sometimes the command seems like it gets stuck despite the player letting go of the key.
This problem can be reproduced if you rapidly tap the block key while wielding any shield.
Sometimes the character won’t raise the shield.
Sometimes the character will keep the shield raised.