Hi Selling_England_by_the_Pound,
One possible explanation for this behavior is known as image persistence. The support article About the Super Retina display and Super Retina XDR display on your iPhone offers this explanation:
"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. Image persistence is temporary and disappears after a few minutes of normal use. Burn-in can occur in more extreme cases such as when the same high-contrast image is continuously displayed at high brightness for prolonged periods of time."
If this is what you're experiencing, it should resolve itself with normal use. If that does not happen, then service may be required. You can visit an authorized service provider listed at Find Locations to have your device examined if necessary.
Regards.