I think K. Rool's Super Armored Belly Dash Attack can go through the shield. I may have to lab this with another person when I get the chance to make sure it isn't just a timing thing, but the whole exchange seems to play out like this:
1. Palu's shield hitbox hits Krool's super armor and cracks it
2. Krool armors the hit, doesn't flinch, keeps going and his belly hitbox gets through to Palu's hurtbox behind her shield and arm.
3. Palu takes damage and knockback
4. Krool has [lightly] damaged belly armor
I'd consider that losing the joust.
EDIT:
Dash Attack Shield can also lose to Cloud's Fsmash. If the timing isn't perfectly timed with the 3 Fsmash hits, the Shield blocks the first hit, but the buster sword is too disjointed for Palu to hit him. Then the next two hits of the Fsmash hit Palu and she takes 24% of the normal ~32% damage.
It's not a straight up loss like vs super armor appears to be, but it's around a 50/50 which isn't worth it given how powerful of a smash attack you're eating if you lose.
With other characters like Link she can still hit him with the shield and interrupt the next hitbox in the one Fsmash move every time, but Cloud's Fsmash's 2nd hitbox comes out too fast; only 4-5 frames later.