If a traveller is praying Qasr while standing behind the Imam who is praying the full 4 units, does this affect the connection of the congregation?
If the traveler is praying exactly behind the Imam of Jama’at, those who pray on his right and left will remain connected with Imam because they are near the Imam, and others who are connected to them will remain connected as well.
If the traveler is praying in the first row away from the Imam, on any side , and he finishes his Salah, those who are away from the Imam will be disconnected if the gap is more than one yard. Many scholars say that one person does not make disconnection, but Ehtiyat ( precaution) is always good. Travelers should not pray in the first row. If they pray in any other row, they will never cause disconnection to others.
Even if disconnection takes place, the Salah will remain valid, but not as Jama’ah but as Furada just after disconnection.