I unfortunately have both image retention and slight burn in on my 12 Pro Max, which was bought on December.
Never had a similar thing on my 11 Pro Max and my XS Max which were bought on day one and have now been given to family members, who also continue to have no issues.
I have the bottom Instagram buttons appear slightly if I use the app for too long.
But they do disappear if I don't use Instagram let's say for 12 hours or so.
The thing that appears permanently, is the shade of the thin horizontal line that's on the very bottom of the screen whenever any app is open.
So whenever I'm not in an app and I am let's say, in the multitasking mode, I can see the shade of that line.
Since I noticed it around two months ago, it has never disappeared completely, so I suppose it's a permanent burn in.
I never really got why this line is necessary in the software, maybe it's to let people know that they can trigger multitasking when pushing up on that spot.
But if there is a spot in the screen that is prone to burn in, it's this line that always appears whenever any app is open.
I have made a research about this problem and apparently, image retention and burn in is possible to happen whether you use light mode or dark mode.
Personally, I use light mode, but I have found other people's posts on Reddit, Apple Discussions etc, that have image retention/burn in issues on dark mode.
It would possibly help if I switched to dark mode, because then, the line would become white instead of black, so maybe, it would disappear after a few days of constant dark mode use, or maybe not, who knows.
I'm not planning to switch up my use because of this, because I hate dark mode.
Also, probably the screen is faulty, so no matter what I do, the problem will probably resurface whether on dark mode or light mode.
I have also read that someone has taken his phone to the service and they put it to the test, but the software found no issues so they did not change his screen for free, instead they returned it untouched.
So maybe I'm screwed this year.
I will attempt to change the screen at the service while the phone is still under warrant and when I already have my 13 Pro Max in hand.
I have posted a photo of the issue, but be warned that if you try to see it through Tapatalk, it does a terrible compression, you will need a computer in order to see it clearly.