What is the ruling on someone who prayed Maghrib with a congregation praying 'Isha as a shortened prayer, then completed his prayer with another congregation with the intention of 'Isha, having missed the first rak'ah?

The majority of scholars hold that a traveler is not permitted to avail themselves of the concessions of travel (shortening prayers, breaking the fast, and combining prayers) until after departing from the populated area of the village or city. A traveler's prayer behind a resident is valid, provided the traveler completes their prayer and does not shorten it. This is something you did not do, therefore your prayer is invalid and must be repeated.