بسم الله الرحمن الرحيم
Shaykh bin Baz was asked:
Some of the people when they pray witr with the imam and the imam gives the salaams they stand and pray another rakah, so that his praying of witr will be in the last part of the night, so what is the ruling of this action?
He answered (paraphrasing)
All Praises belong to Allah,we do not know of any problem or sin in doing so and it is affirmed for him that he prayed with the imam until he completed the prayer, because he prayed with him until the Imam finished, and he increased by a rakah for a legislated benefit, meaning: so that he could pray witr in the last part of the night, so there is no problem with this and it does not take away from the fact he prayed with the Imam until the imam finished.
من كتاب الجواب الصحيح من أحكام صلاة الليل والتراويح للشيخ عبد العزيز بن باز ص 41ه