بسم الله الرحمن الرحيم
It is not compulsory to offer witr salah immediately after Isha salah rather its mustahab time is the time of Tahajjud provided one is sure to get up timely. Thus you cannot object Imam sahib if he does not offer witr salah after Isha, so this cannot lead to make his Imamat makrooh.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.