Shaykh Abdul-Muhsin Al-Abbaad was asked:
“If the Imam in Witr praises Allah, the Exalted, then sends peace and blessings upon the Prophet (ﷺ), is is a must for the one behind the Imam to send blessings upon the Prophet (ﷺ)?
“If the Imam says: “May peace and blessings be upon the Prophet…” he says Ameen because it is a supplication. Likewise, if he praises Allah, he does not say Ameen because it is not a supplication. However, he also praises Allah like saying Subhaanah for example”. End translation. [Sharh Tirmidhi no. 82]
Faisal Ibn Abdul Qaadir Ibn Hassan