This is a hard one as if I’m correct you might have to import it as a obj from blender. Also are you using the Rbx Animations addon?
Either way thse are a few things I can think of to help you:
Import your export into blender and see if you get the same issue in roblox.
Make sure all the bones are linked as one skeleton and not peranted as joints can be flipped due to peranting not being exported properly.
Check your FBX export setting, especially with the “armeture” section as your bones might be exporting on the wrong axes.
For some reason I can import the animation and it properly imports using the plugin, but then when I save the keyframe sequence and try to then use it, it doesn’t import anything into moon animator, it’s just blank.