According to the Minecraft wiki page on Axes, axes are supposed to disable shields 25% of the time, "plus 5 percentage points per level of Efficiency on the axe, plus 75 percentage points if attacking while sprinting." But in a vanilla 1.16.3 server, axes break shields 100% of the time, regardless of those factors.

To reproduce this, have one player block a shield. The other player may stand still and click once with an axe, and the shield will be disabled 100% of the time.

video of bug