Is saying "Haqqan wa Sidqan wa Ya Allah" (Truly and Truthfully and O Allah) when the Imam praises Allah in the Qunoot considered an innovation (bid'ah)? And what should be said if it is an innovation?

Al-Nawawi said in Al-Majmu': The worshiper should join the Imam in saying: "For You are the One who decrees and none decrees against You," or remain silent. Joining is more preferable because it is praise and remembrance, and saying "Amin" is not appropriate in this context.