That's odd, I found the same thing to be true of spearmint ess oil. It was absolutely heavenly in the bottle very candyish, but only a whisper of itself in cold process soap. Both faded for me more and more as time went by too.
Very strange - I don't have that problem with Spearmint either.... mind you it is lighter than Peppermint and I love blending them together with Peppermint at a lower %age than Spearmint. Smells like a candy cane.... yummy