On iPad Pro, does some apps stay full screen even when working in a 2 app side-by-side mode? Because Pokemon Go has a bug on the iPad version and I cannot see the number of Pokeballs remaining to catch a Raid Boss (due to aspect ratio of iPad about 4:3, not the same as iPhone's 16:9), so I want to use the 2-app-side-by-side method to see if I can somehow change the aspect ratio of the app.

So I either run Pokemon Go and slide in a Safari, or run the Safari and slide in Pokemon Go.  (By running Pokemon Go, and then slide from bottom up of the screen to see the dock, and then touch-and-hold an app and move it to the edge of the main screen area).

Either case, I see Pokemon Go running as a full screen app.  Is it true? Some apps cannot run as a side-by-side app but must remain full screen?
Yes, that is true. The app must be compatible to run side-by-side - and if the app is not, there's nothing users can do about it other than encourage the developers to add the support.