I used a Repulsing Seal trap on some Shadows. "Immune" popped up above their heads when it went off, and in the battle logs at the bottom right, I see their Fortitude defence show up as "Imm.":
I'm puzzled by why. I know that the Fighter's Knock Down ability works on Shadows, which - like Repulsing Seal - is also a "vs. Fortitude" ability that causes Prone. The Bestiary page for Shadows clearly shows a finite Fortitude defence and doesn't show any immunity to Prone. So what's going on? Is this a bug? A hidden special-case immunity just to Repulsing Seal that isn't indicated anywhere in the Bestiary? Or have I missed something in the Bestiary that should've clued me in to this immunity?