who was the archon when mavuika ended her first life

Mavuika was the Pyro Archon when her first life ended, so the answer is Mavuika herself.

In Natlan’s lore, she is described as the latest Pyro Archon, and the sources I found identify her as the Pyro Archon of Natlan.

If you meant the archon of a different nation at that moment in the story, tell me which quest or scene you’re referring to.