I thought it was intended that when a player enhances a religion, no new pantheons can be founded. I've always seen pantheons being founded way after I enhance... 
The most annoying thing is when I painfully convert an entire other civ (say on a different continent) who hasn't founded either a pantheon or religion to my religion, only to have it overthrown by that civ founding its own pantheon, then all of its cities are half his new pantheon, half my religion, and... I lose all the pressure seeing as my religion is no longer dominant in any of his cities.
Is this a bug?

The most annoying thing is when I painfully convert an entire other civ (say on a different continent) who hasn't founded either a pantheon or religion to my religion, only to have it overthrown by that civ founding its own pantheon, then all of its cities are half his new pantheon, half my religion, and... I lose all the pressure seeing as my religion is no longer dominant in any of his cities.
Is this a bug?
