Problem with a mirror object

I'm not familiar with $$anonymous$$ax, but I would first try to apply the mirror modifier in $$anonymous$$ax and then export it to unity to see if that is the problem. If everything looks fine in $$anonymous$$ax, then I'm assuming the mirror modifier wasn't applied before you exported it.

In Blender, there is an "Apply $$anonymous$$odifier" checkbox, which you can check to apply any modifiers that have not already been applied, this is cool if you want to continuously make changes with the modifier still unapplied even after applying it for the export to see the result in Unity. I'm sure $$anonymous$$ax has a similar feature when you export and if it doesn't, I would duplicate your object and apply the mod to the duplicate.