Its not mauve. It bothers me.

Mauve is one of those colors that can actually run a gamut. The flower that mauve is named after, the mallow, is actually very similar in color to the belt:

The dye "mauve" or "mauvine" is actually a very bright, saturated pink-purple.

And the wiki page has a whole list of "mauve" colors, all of which are very different. So there's not really one mauve, and the belt technically is mauve! Just not the maroon-y type of mauve that, say, your outfit includes.

It looks mauve to me, and matches a Pantone color with 'mauve' in it. There are a lot of color variations in Subeta, and this is not a big one, IMO.