My Youtube app won appear anymore if I click home. What's wrong? Previously, when I click home on my s9, if my youtube app is playing a song for example, my phone would go to home but it would still display the app like this. 



And when I click on it, it would maximize like this.



It's not PiP (I think), because I don't deliberately set the app like this, it just automatically does it when I click the home button. I could even browse chrome and open other apps and it would still be there on the lower right..

But now, suddenly, it's gone!
When I play something on the app and then click home, it just closes. The preview window doesn't appear anymore and nothing is playing.

I'm just wondering what I did wrong and how I could fix it. I don't like using PiP because it's not automatic and I like how it would automatically just preview itself before.
It depends on where you are from. What you're seeing is the PIP mode of YouTube.
For most parts of the world, it is only included with YouTube Red. So, maybe you had a YouTube Red/Pro subscription or Trial which has expired. 

YouTube also enabled the PIP for free to all users in the US. That is, even those who don't have a paying plan. So, do you happen to be from the US? Plus, they could be testing the free PIP support in in other regions too.