Ok, for anyone running into a similar issue in the future:

TL;DR: Use animation.addByPrefix, and make sure your animation frames are separate regions within the packed png (walk-01, walk-02, etc).

I gave up on trying to use animation.frameName for this, no matter what I do I cannot get the correct region to load in. The only method that I could get to work with fromLibGdx was animation.addByPrefix.

In my existing game each region within the packed image is a spritesheet, rather than an individual fram and the game adjusts the frameHeight / frameWidth of the sprite for the animation, unfortunately this method doesn't work now since addByPrefix adds an entire region as a frame so I am exporting all of my animations as individual frames to be packed by texturepacker.