Can't found religion

Don't know if this was answer or not or if this is a bug or gameplay feature but I can't found a religion. I'm playing India on a Duel map with 10 civilizations and no city-states with Domination victory only. I was the first civ to found a pantheon and like 25 turns later, Christianity was founded by Byzantium. I reached over 300 faith and got a Great Prophet but he couldn't found a religion. I hovered my mouse over the faith icon and it says "Religions that still can be founded: 0". So I thought there were six religions already founded and when I checked, Christianity was the only religion founded. Is this a bug or a feature? I usually play on standard maps so I don't know if a duel map affects the number of religions founded.

I believe that always 1 civ can not found a religion.
So 2 civs (duel) --> only 1 religion can be founded.
Not sure, but that seems plausible to me.

Civ5Worlds.xml states that, for a Duel map, <MaxActiveReligions>2</MaxActiveReligions>.