If you are unfortunate and receive a notification (i.e. Population Cap) while an enemy reaches a new age, then you won’t receive the banner notification.
At 1917 - the opponent hit Castle Age but the overlay was not shown (pulled from DeMusliM’s YouTube):
I thought I’ve noticed ( or didn’t notice rather ) this a few times in games I’ve played (but blamed it on the fact that I probably just tuned it out from other things happening), but noticed it in this video.
I believe this is due to the shared banner notification component - I think a good solution to this would be to queue notifications if the message differs from the current notification.