Greetings, oldsquid.
I see that you're having problems multitasking on your iPad Pro. When you select another application to use, the original application minimizes. I'm happy to help.
Use Multitasking on your iPad -- This article has information for using multitasking on an iPad with iOS 12 installed.
Restart your iPhone, iPad, or iPod touch -- If everything was correct, restart your iPad. Once it's returned to the Home screen, test the issue.
How to back up your iPhone, iPad, and iPod touch -- If restarting your iPad didn't resolve the issue, please be sure to have a fresh backup to keep your personal information safe.
If you can't update or restore your iPhone, iPad, or iPod touch -- Even though the title of the article isn't specific to your issue, the section titled "Put your device in recovery mode and set it up again" will walk you through all the steps to reinstall the iOS software on your device. Test the issue once again.
Let me know how that goes, and take care!