The tile yield preview before placing after a growth event is not what you actually get when it comes to happiness. By placing an improvement the happiness yield actually is reduced or goes away completely. I think this might be tied into an appeal system, but how it works is not explained very well. It would be nice if there was an appeal lens like in Civ6 so we could better understand what is going on. So, if you are like me and you thought that you could boost happiness after an age transition by working farms or mountains, think again.