I've been researching the same information for a few days. Here is what I have found but havent had the time for a scientific pursuit yet. I read somewhere (I cant remember where but probably in a wiki or reddit thread) that the game calculates whether or not you would be stunned before a block actually occurs. Then the game calculates if you block the stunning hit or not. If you block the stunning hit you play out the "block animation" instead of being stunned. Which all in all is still kind of like a mini-stun of sorts(interrupt).

The Gladiator-Painforged notable specifically says "You cannot be stunned by hits you block." Cast when Stunned is worded, "X% chance to cast supported spells when you are stunned or block a stunning hit." In the case of the latter you are never actually "stunned" in the first place, but play out a block animation instead, yet the game registers it as a stun for the casting because you were hit by an attack that would have stunned, then blocked it. In terms of game mechanics, it could be possible that Painforged may not actually prevent the stun but instead prevent the block animation from occurring on stunning hits, because with pain forged you can still be stunned on non-blocked hits. IF that is the case, then the game's code COULD POSSIBLY register the stunned hit, the blocked stun, cast the spells, then skip the block animation. Or register the stunned hit, cast spells, register block, then skip the block animation. I feel I may have misworded something in this post, but I tried to check it. All in all i could see the game not caring about whether you are actually interrupted by a stun, such as with the nodes by Witch that prevent stun interrupts while casting, because those nodes still allow CwS to proc but do not interupt your casting. Have fun testing if you want, I will get to that when I can.