How does Wooga decide who gets a Detective Needed side game vs. A Fairground Fortune side game?

Of course there's a reason.. my guess is, unless a person has all of their tracking cookies turned off, then the game is tracking the Analytics of a person's game play ..for example, If a person's game play indicates they are collecting Snippets then that is probably the focus of the side games presented to that player.

Where do I find the "tracking cookies "?

If using an Android, you can go to settings and find the listing for apps, scroll down the apps until you find June's Journey, tap on it and find a section pertaining to app permissions.

If you are using a Windows-based system I believe you will find Cookie controls in your browser settings..I assume an iPhone would be similar to an Android where you would look in the settings to find your apps and app permissions. I am not an expert, by the way.