• I had a couple games where I selected normal difficulty or above and the AI was more proactive than it is on casual; advancing in age ahead of me and so on rather than waiting. Recently my past few games or attempts at games the AI has acted as if the difficulty is on Casual instead, despite selecting Serious. 
  
  
• Steps to Reproduce: Completed a game on normal difficulty where the AI acted proactively with regards to culture selection, started not doing this on a following game despite same or higher difficulty. I guess?
  
  
• Reproducibility: unsure
  
  
• Attachments: I can try and upload saves later perhaps if they're necessary, but I'm playing on a different computer than the one I'm on currently. 

Confirming, the selector (drop down menu) itself is bugged.

Despite the actual difficulty selected if you don't touch the menu it will always be the easiest. If you actually select something the difficulty will be applied correctly. 

Can confirm. I tried to click through every difficulty and then landed on the difficulty I want, the AI will advance into next Era properly.

In addition, here is a reddit post that also pointed out this problem and provided a working solution; and this is another forum post which also confirmed clicking through every difficulty then choose the difficulty you want will land you on the "real" difficulty.