Hey there VariantXYZ,
Thanks for reaching out to Apple Support Communities. I understand you’re having some issues with how app switching works on your iPhone. I’ll be happy to help.
If you have an iPhone X or later, or an iPad, you should need to swipe up from the bottom to the middle of your screen and hold until you see the App Switcher. You can read more about this here:
Switch apps on your iPhone, iPad, or iPod touch
Is this only happening when holding your iPhone in landscape? If so, try restarting your iPhone and see if this has any impact on this:
Restart your iPhone, iPad, or iPod touch
If you continue to notice the issue, remove any case or screen protector you may have on your iPhone and try again.
If you still notice this happening, backup your iPhone and reinstall iOS using these steps:
How to back up your iPhone, iPad, and iPod touch
If you can't update or restore your iPhone, iPad, or iPod touch
I hope that helps.