These are two separate Hadeeths; the one about Isha and Fajr, which is authentic and the other one is regarding taraweeh with the Imam which is also authentic. One should pray until the Imam concludes with witr in order to gain that reward.

the hadith which says if you pray Isha & Fajr in Jama’ah, you’ll get reward of praying all night. someone said it’s only applicable if you pray witr also in Jama’ah with imam or else you won’t get that reward. if the imam also doesn’t pray witr after Isha, only then you can delay witr. otherwise you must pray it in Jama’ah to get that reward.is it right?