I stay in Kolkata as a muqim. When I was in Delhi as a Musafir, I board the flight after praying Maghrib and waqt for Isha started in Delhi itself when I was in the flight. I decided that I would pray Isha after reaching home (Kolkata) at around 10 PM. If I would pray Isha in Delhi itself, I would pray Qasr. Now I am back to home, do I need to pray Isha as Qasr or as a muqim (4 rakah) for that Isha which I did not pray in Delhi.
بسم الله الرحمن الرحيم
In case of offering qasr or full salah, the last time is taken into consideration, not the starting time. Hence if you reached home at a time when the time of Isha was remaining then you will have to offer full salah.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.