I contacted support to see if it was a bug or by design, and got this reply. They weren’t able to go into specifics but this at least helps us understand how it works.
“Upon entering a new era, you retain an amount of gold and influence proportional to your civilization's per-turn earnings at that time.
So, in conclusion, when you build a robust economy in your current civilization, you'll maximize retained resources when advancing to a new era. I hope this clarifies things further for you.”
“Upon entering a new era, you retain an amount of gold and influence proportional to your civilization's per-turn earnings at that time.
So, in conclusion, when you build a robust economy in your current civilization, you'll maximize retained resources when advancing to a new era. I hope this clarifies things further for you.”