Why does maximising the volume on a Fender Mustang Amp increase the perceived brightness?
One reason may be the loudness curve that represents how sensitive we are to sounds at different volumes. Compared to midrangey sounds, we are more much more sensitive to bass and slightly more sensitive to treble sounds at higher volumes.
Another reason would be that amplifiers tend to distort more at higher volumes. This is true even of amplifiers that are set to 'clean', i.e., to not deliberately distort. These distortions will create more higher harmonics and a brighter sound. If you are using distortion deliberately the same principle may apply.
On the guitar (rather than the amp), maximizing volume can give you a brighter sound due to capacitance effects. This seems much less likely in an amplifier though.
Of course if you're seeing a dramatic increase in brightness with no obvious increase in distortion, there's a possibility of a bad design or a faulty amp.