Was the traveler right to shorten the prayer when he prayed Dhuhr with a congregation without knowing whether they had completed or shortened it, and then prayed Isha shortened with a congregation performing Maghrib in their last rak'ah?

If a traveler doubts whether the Imam is a traveler or a resident, there are two scenarios: 1. If there is an indication that the Imam is traveling, such as his presence in travel locations or the appearance of travel signs on him, the traveler may shorten his prayer. 2. If there is no indication, the traveler must complete his prayer with four rak'ahs, because the default is completion.

Based on this, if the traveler shortens his prayer without knowing the Imam's status and there is no indication, it is safer to repeat the prayer in full. It is permissible for the traveler to shorten the 'Isha prayer behind an Imam who is praying Maghrib, or to complete it with four rak'ahs.