iPhone 8 screen not closing correctly after screen replacement

that chip in the lower corner of the screen could be the issue, but usually non-OEM manufacturers are pretty good about matching physical dimensions. do you have a caliper/micrometer to get an exact thickness measurement of the two?

that said, most of the time when i see gaps like this between the display and the back casing it has to do with the side guide/grip fingers on the display. in the first picture showing the side, you see the rectangle flange with the two cutouts (this is for alignment of the display frame and there's a little bump that "connects" with a bracket on the inside of the back case to hold the screen down) and the two (of 6 total) screws that hold the metal shield to the display frame. what i've found in most cases is the flanges for the alignment or screws are slightly bent and prevent closure or don't grip and don't hold it tight.

not saying this is your issue, but this is what i've found when i have seen this scenario.

hope it helps