Just bought and opened another treasure chest today. Found there new (like, absolutely new - there aren't even any place in Gallery for it) legendary blessing. And, considering, that it isn't mentioned in the gallery, I'm concerned. Will that become a new (even if rare) bug, where we will recieve non-released yet content in treasure chests? And, that blessing was with "add 2 dice for check to defeat villian or henchman"-type Prayer (there was creepy-looking chalice for the art, and i don't really remember which god it was, but most likely Rovagug)

Don't worry too much about it.  There have been reports of a handful of rogue treasure cards (cards that may be obtained in treasure chests, but may not necesarially show up in the gallery).

 

I think it is possible for them to still show up during play even if they seem "invisible".  Otherwise, whenever the next patch happens, whatever card you got might make their official debut.