Hello OndrejKlaban,
I understand you would like some help with your iPhone regarding portrait selfies looking washed when comparing them to the same photo taken using a different iPhone model. I'm happy to help you out with this.
Try turning off your Smart HDR settings to see if that makes a difference in how your front-facing photos come out. You can find steps on how to do this in Adjust HDR camera settings on iPhone, specifically this section:
"On iPhone 11, iPhone 11 Pro, iPhone 11 Pro Max, iPhone XR, iPhone XS, and iPhone XS Max: Go to Settings > Camera, turn off Smart HDR. Then from the camera screen, tap HDR to turn it off or on."
It may help to take photos in Portrait mode as well. Here's more information on how to do that: Use Portrait mode on your iPhone. In addition to that, make sure your iPhone is updated to the most current iOS version: Update your iPhone, iPad, or iPod touch. Also, clean the front camera lens with a microfiber cloth.
Thanks for using Apple Support Communities.
Take care.