Animation Clip frame-length nearly triples in Mecanim

I've encountered a weird behaviour with the animation system.
I'd like to play an animation clip in Mecanim at the imported frame-length, which is 15.

For some reason, the animation's frame-length seems to almost triple in Mecanim to 42. This is true in-game and when viewed in the Mecanim Animation-State inspector, irrespective of the game's frame-rate. See image below:



When we preview the imported animation asset in the inspector, we see the correct frame-length:


This seems to happens in an empty project. The Animator, Animation State, and Timescale are all set to 1.
This also isn't specific to one a specific clip or a particular model.

How can I get the Animation clip to play at its imported frame-length in Mecanim?

It would be great if we could get the animation to play as it's viewed in the asset import inspector.
Getting the frame-length right is important, as we're designing particle systems to sync with the animations.

Any insights into this would be greatly appreciated.

I'm using Unity Version 2018.3.0f2