What you were expecting to happen, and what actually happened:
I was expecting enemy shiny troops in PVP to work reliably. Instead, as you can see in the above screenshots, they appear shiny when you click on them but not shiny in the preview card, with different mana costs listed.
How often does this happen? When did it begin happening?
When playing PVP.
Steps to make it happen again
Play PVP until you encounter a troop that can be shiny. The above screenshots were taken in Aidania (brown restriction) and Summer Isle (Suncrest restriction).
As you can see, one of them appears shiny (only when clicked on) but the other does not. I have both at shiny level 1, so I assume that the player that I was facing does not have Keeper of the Paths at shiny level 1.