Why is ad still briefly playing after a restart? I’m playing this game on an iPhone and, every 3 rounds there’s an Ad playing that you can usualy skip after 5 seconds. 

However, it takes shorter to just kill de app when the Ad appears and launch it again than to wait those 5 seconds. + I don’t have to see the ad this way. 

However, after the app relaunches, the ad still plays for a second before the app launches normally. It’s like that part was cached or something. 

How can iOS allow that? It’s clearly not something the app developer intended, or that the devs could even control. 

I should mention though, the ad only plays if I quickly relaunch the app. If I wait a bit before relaunching the app, I no longer see the ad.
iOS has a feature to make "unpausing" apps seem faster when they are not in memory anymore. Your device keeps a screenshot of your app and if it has to be purged from memory (put on pause) it will show the screenshot until the app is ready. 
This usually works very well but can look unexpected when you force quit the app which leads to it not taking off at the same point but showing the initial screen again.