Date & Time: 6/1/2022, 4:45 PM PST
Frequency: Anytime
World name: Cactuar
Character name: Flufferbutt Butterbuns
Class/Level: PLD lvl 74 - 90

Steps:
1. Engage any enemy that is capable of applying Stun to you. The tethers in Dragonsong's Reprise(Ultimate) during Strength of the Ward is an example.
2. Use Shelltron/Holy Shelltron before you get stunned.
3. Any damage you take while you are stunned and also while you have Shelltron/Holy Shelltron up will not be blocked.

I never thought about it, but it does seem pretty logical that you can't raise your shield to block something while stunned.

You cannot block or parry while stunned, just like you cannot auto attack, block, or parry something to your rear or flank if you are bound. That’s intentional.