Hello cosmin298,
Thanks for posting in the Apple Support Communities.
It sounds like what you're experiencing may be expected to occur with the Super Retina XDR display on your iPhone 12, but let's confirm this. If what you see appears to be a transparent view of what was previously on your iPhone screen, this could be expected to occur with these displays as outlined below. You can also locate additional details on these displays here: About the Super Retina display and Super Retina XDR display on your iPhone - Apple Support
OLED technology
The Super Retina and Super Retina XDR displays use organic light-emitting diode (OLED) technology. Super Retina and Super Retina XDR includes further advancements over traditional OLED displays to enable an incredible viewing experience, for the first time rising to the standards of iPhone.
OLED technology delivers an incredibly high contrast ratio and high resolution. And with no backlight, OLED emits light through each pixel, allowing for a thinner display. The Super Retina and Super Retina XDR displays overcome challenges with traditional OLED displays with their high brightness, wide color support, and incredible color accuracy.
If you look at an OLED display off-angle, you might notice slight shifts in color and hue. At reduced display brightness levels against black backgrounds, you might notice a slight blur or color change while scrolling. These are characteristics of OLED and are normal behavior. With extended long-term use, OLED displays can also show slight visual changes. This is also expected behavior and can include “image persistence” or “burn-in,” where the display shows a faint remnant of an image even after a new image appears on the screen. This can occur in more extreme cases such as when the same high contrast image is continuously displayed for prolonged periods of time.
We’ve engineered the Super Retina and Super Retina XDR displays to be the best in the industry in reducing the effects of OLED "burn-in." This includes special algorithms that monitor the usage of individual pixels to produce display calibration data. Your iPhone uses that data to automatically adjust the brightness levels for each pixel as needed to reduce visual effects from "burn-in" and to maintain a consistent viewing experience.
In addition, all displays, including OLEDs and LCDs, might be susceptible to reduced brightness levels as the display ages over time. This can occur on any consumer-electronics product.
If that seems to align with what you're experiencing, this would be expected to occur. You can locate helpful tips in the resource, so be sure to review those if you haven't yet. Let us know if you have any additional questions.
Have a great day!