Very very very short…
It looks like this in Blender. I export it as a FBX. Preferrably one can set the scale to 0.5 and then import it as MM so that one can keep the Unit Scale to Blender’s original values.
It looks like that in Roblox Studio
And that is the issue.
I played around with this, but I couldn’t seem to get it right either. Especially Secondary bone… it doesn’t seem to do anything at all.
The bones are just wrong.
Nor is it using this:
It’s using Motor6D
Maybe I’ve figured it out.
It didn’t have these modifiers.
But there’s still a couple of issues. In short idk if Roblox supports bones parented to meshes.
RootPart is ruining it
gltf works fine though. But it isn’t creating the Bones.
Hey! Sorry to hear you’re experiencing issues. I’ll try my best to resolve.
The mesh output in the preview does seem strangely rotated 
To help us repro, would you be willing to share a bit more info? What does the model hierarchy look like currently vs. what do you expect?
Feel free to DM me more details and/or the .blend file as well and we can try to repro the issue.