Sometimes, FFD key vertices aren't being exported into the JSON.
I noticed this happens when the vertices are the same as in the setup pose.
It causes interpolation to stop working (as it would if the same thing happened to any other key, I guess?), especially in a loop where it goes from setup pose vertices to a deformed one and then back to setup pose.
I think interpolating from a vertices-less key still kinda works. But interpolating into one doesn't... I think.
This is a 1.7.12 bug though. Not sure if it was fixed in .13 or .14
EDIT: In the editor, not on export: the curve on said vertices-less keys also don't persist very well. Doing some editing on the key or moving it or pasting it or something seems to obliterate it.