screen replacement - only some screen

I’ve been trying to replace the screen on my iPhone 12 mini. I bought two replacement screens from Amazon and two from eBay. Here’s what happened:

• One screen had a good display, but the touch didn’t work.
• One screen showed nothing at all—completely blank.
• One showed the Apple logo, then went black.
• One just displayed colored lines.

At this point, I was going crazy.

Then I happened to get another iPhone 12 mini. I tested the screen that was previously totally blank on the new phone—and it worked perfectly! The display and touch were fully functional. So I went back and tried the same screen on the original phone again, but still just a blank screen.

I should mention: the original iPhone 12 mini did have a semi-working screen before I started. The display was mostly fine, but it had two vertical green and yellow lines running top to bottom.

So now I’m wondering—what could be wrong with the first iPhone 12 mini that’s preventing it from working with the replacement screens?

Would you like help diagnosing the issue, too?