So I tried the new snapshot, and made a new creative world in order to see what the creaking is like.

Placed down a creaking heart, nothing happened. It didn't even glow. I then set it to night. The block was still inert. Went into survival mode. Nothing happened. 

I even set the nearby area to the pale garden biome to see if that it'd fix it. It did not, even when trying the above things again with this condition in place.

Edit: turns out I was using it wrong; I thought creaking hearts worked on their own. No, apparently they needed to be placed in a certain way in order to work.... they don't work just by themselves...

Either I'm using it wrong, or it's broken. (I'm also certain it's not because of the mob cap, because /kill-ing the nearby hostile mobs did nothing to help this work either.)