iPhone cannot close other apps when facetime is connected

Here is what Apple says about the apps in the background. Apple text is underlined and bolded.

After you switch to a different app, some apps run for a short period of time before they're set to a suspended state. Apps that are in a suspended state aren’t actively in use, open, or taking up system resources. With Background App Refresh, suspended apps can check for updates and new content.

If you want suspended apps to check for new content, go to Settings > General > Background App Refresh and turn on Background App Refresh. If you quit an app from the app switcher, it might not be able to run or check for new content before you open it again.

iPhone X never ran iOS 10. It is different than the other phones.

They do not close when you are on the phone either. It is just the way it is.