Medium-sized slimes don't deal damages to shields.

Can you add a video with your f3 on?

A video added.

this bug affects all damage sources that deal less that 4 points (2 hearts) of damage, not just medium slime attacks: https://minecraft.fandom.com/wiki/Shield#Defense

What difficulty are you on when reproducing this issue?

Medium slimes deal one heart (2 damage) on Normal difficulty, and one and a half hearts (3 damage) on Hard. On normal difficulty durability is not lost, because the attack does not deal enough damage (per this comment by @unknown). When compared to hard difficulty, the damage is high enough (3) for durability to be lost.

Here is a demonstration: