Thanks for posting! I understand you're seeing some display flickering on an iPhone 11 Pro Max.
It sounds like it happens when using multiple apps. Have you tried removing any cases and screen protectors to see if that makes a difference?
Have you updated to iOS 13.1.2? How to back up your iPhone, iPad, and iPod touch and Update your iPhone, iPad, or iPod touch
If you find the issue still occurs after updating and testing without cases/screen protectors, you might try restoring your device to factory settings to test it while set up as new. Be sure to back up first using the steps above: Restore your iPhone, iPad, or iPod to factory settings
I hope this helps get you started. Take care.