I just noticed that. I was thinking it was a bug.

But look like a game mechanic, nice one.

The more you wait before to get the stars, the less it give fame.

You get that too ? (else, I have a bug on my game!)

Yes, it is a nice mechanic to stop people from stalling an era too much to get all the stars.

It's intentional, not a bug. There is a minimum cap on it afaik, but yeah it's basically a punishment for people who stay in an era for too long.