A learner removes tanwin from صَالِحٌ only because it is a name. What is the mistake?

Causes of Ghayr Munsarif: Proper Names and Foreign Origin

• صَالِحٌ is an established declinable form.
• Every name must take fathah.
• صَالِحٌ is a preposition.
• Tanwin is never used with names.