[font appearing as a different thickness]

Does this happen exclusively in paint.net?

Yes.

[font appearing as a different thickness]

I have one font from a family of fonts that appears as the wrong thickness--it appears the same as one of the other fonts in its family. I don't have this issue in other applications or with other fonts.

 

This font is the OpenType font "Fluctuation". The "book" font thickness in the family appears as the "regular" thickness; however, if I uninstall the regular font, then the book font appears correctly. When I reinstall the regular font, the book font again appears as wrong.

 

All of the other fonts in this family appear correctly.