Assalamu alaykum, in most of the masjids in our country the imam make congregational dua just after salah. In that case is it must to leave the masjid immediately? But sometimes it is difficult to leave from first rows while many people are sitting behind and some of them completing their missed portion. In that case if i remain sitted in my place and complete my masnun adhkar without joining them in congregational dua, is it ok?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Collective supplication after each prayer is a matter that is not in conformity with the Sunnah and doing this on a regular basis renders it an innovation.

However, you are not obliged to leave the row or the mosque, and there is nothing wrong in staying in your place and completing the Thikr (mention of Allah) that is said after the prayer.

Allah knows best.