Because when it stands alone, there is nothing to cause it to mutate.
A common reason for it to mutate is when it follows a feminine noun or when it is referring to someone or something feminine:
- Y gadair fwyaf yn yr ystafell yw hon - cadair (chair) is feminine (you can tell that because it has mutated after y), so mwyaf must take a soft mutation.
- Y siop fara ydy'r un fwyaf yn ein pentref - 'The bread shop is the biggest one in our village'. yr un fwyaf ('the biggest one) is referring to the siop. Siop is a feminine noun, so mwyaf has had to mutate to fwyaf.
Is the pronunciation on the TTS right on this one, it sounds like mwy-ar or is it a more fluid mwya