It turned out that for all those years the way ModelConverterX handles the mipmaps of textures was not really correct. So in the last version of ModelConverterX there are a number of changes that improve this. These include:

• Mipmaps are read correctly from DDS and DXTBMP files, instead of being calculated internally by ModelConverterX.
• Mipmaps are saved to DDS and DXTBMP files based on the data that was loaded, instead of being calculated by ModelConverterX. This means that if you modified some mip levels that data is not being lost when exporting a texture file from ModelConverterX.
• The Texture Converter form has a dropdown menu now where you can select which mip level you want to see, that way you can inspect the different levels.
• And last, but not least, the preview in ModelConverterX uses the mipmap data correctly now when rendering the preview of the object.

The video below also shows most of these changes.