It becomes faint to all who witness the interaction.
Jeremy Crawford, athe lead D&D 5e game designer, tweeted the clarificationintent behind the rule:
Physical interaction with minor illusion reveals it to be illusory to anyone who witnesses that revelation.
The next paragraph - "if a creature uses its action to examine the sound or image..." - implies examination withoutwithout physical interaction, which requires a successful Intelligence (Investigation) check and works for a specific creature only (the one who succeeded the check).