I've tried this Tutorial
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I looked into it since I also wanted to utilize ffxiv animations.
The reason you are getting the 'buggy' animation seems to be that the animation tracks are getting applied to the wrong bones in the skeleton.
The havok2fbx tool was not taking into account that ffxiv animations utilize hkaAnimationBinding to specify which bones the animation tracks apply to. It was assuming a 1-to-1 mapping of Track-Bone when ffxiv havok files seem to have more bones in the skeleton than tracks in the animation, so no 1-to-1 mapping.
I released a modified version of the tool here that should address that problem:
First time using GitHub to make a release so let me know if I screwed it up somehow.