So finally got a pet to a point of being able to test to see if anything had changed. While I'm not using a yog pet the system is the same for all pets. (I'm using a Blue Lep) I can say there has been no changes with this I had no issues in unlocking a buff for a pet. I also did not need to go to a pet center to do any of this, did it all right in my inventory! (Only had to go to the pet center to squish a couple of poor pets to get the essence -
for the greater good right?)
Going into the Show Pet Status in my inventory
This brings up the pet status like this
Currently, I cannot afford it, as I don't have the essence on me, but it shows that it is ready to unlock. (which, from your images, was the point that you were on)
I have made sure that I have the essence in my inventory, not in storage and made sure that I had enough ped on my card for the price of doing this. I refresh the page by reopening (or pressing info and going back to the passive that I want to unlock)
Now it shows me this
With the 3 ped that it takes to use, and the 15 essence that it needs the cannot afford turns to unlock in orange.
When I click unlock, it pops up with the Confirm Unlock
Then it is unlocked but still needs to be activated.
So pressing Activate will enable the passive buff on the pet.
Now my pet has the buff enabled and can carry on.
While the pet might be different, the system is the same, there has been no changes to this!