Maybe it is the animation? Maybe try exporting with just a static pose and see if the bones are in right positions?
I thought I had a model with floating bones to test, but it turns out I don't, so I cannot currently confirm whether or not floating connected bones work. I would see if you can get a static pose working, and then if that doesn't work maybe open a Github issue?
I'll make a quick model to test later, and then I'll report back with my results.
Nice model by the way!