I noticed several enemies that have hidden resistances or immunities, these are all NPCs. For example Gulfaryc in the Subterranean Temple seems to have resistance to perception afflictions, and an animal companion in the same encounter has resistance to intellect afflictions. Neither of those resistances are visible when you hover over them.

 

(blinding strike here is causing the disoriented affliction instead of the blind affliction) 

 

Magranite Champions in the Steel Preacher bounty also have hidden poison immunity.

Greetings Octo1,

 

And thank you for taking the time and bringing this to our attention We'll go ahead and add this to our data-base for our developers to investigate.

 

Cheers