There are a couple reasons this can happen, specifically texture import settings in unity, or could be an issue related to your specific encoding setup. Likely, you should check the VAT texture compression in Unity is set to RGB 16 or Automatic (differs between Unity versions) and check that it is imported with max size matching the real dimensions of the export
Its a good addon, very intuitive, although I'm having and small issue with the values when I import the model to unity, it seems all my vertex are displaced and it doesnt keep the shape correctly