The ruling on one who prayed Maghrib behind a man who was praying 'Isha'

I was traveling and I found a person—may Allah honor you—in the restroom, and we agreed with him,And I said to him: Did you pray Maghrib and 'Isha'?He said: No, and we were between Maghrib and 'Isha'. When we entered the mosque, we found a person praying, and we did not know what he was praying. We entered with him because we wanted to pray Maghrib, but he was praying 'Isha'—he was praying it as four rak'ahs. So we prayed with him from the first rak'ah. We did not sit in the third, or I sat but I doubted when the one next to me, who entered with me, stood up with the imam for the fourth. I doubted that the imam was praying Maghrib, so I doubted that I was distracted and did not notice and did not know that I was supposed to sit in the third. In any case, we prayed four rak'ahs with him. When I finished the prayer, I asked the one next to me who entered with me,So he said: that he had prayed four rak'ahs.He said: What is the ruling? We exercised independent reasoning and we repeated the Maghrib prayer as three rak'ahs, then we prayed 'Isha' as two rak'ahs after that. What is the ruling? May Allah reward you.
There is nothing wrong with this.Meaning: What you did is valid. But if you had intended Maghrib, then when he stood up for the fourth, you should have sat down, recited the tashahhud, and said the taslim. Then you should have joined him in the remainder of the 'Isha' prayer and completed it. This is the way of the issue. But you repeated it, may Allah reward you with good.