Inverting when turning on and off?

So dropped my phone a few months ago, and it worked fine until recently. Whenever I put it down, the screen would light up white across the cracks, even if it’s off. I wasn’t to concerned, got a heavy duty case and left it be.

Couple days ago, my phone began to invert the colours randomly, but only for a split second when i turned it on and off. I’ve double checked through settings that inverted colours were all off. Its not bothering me or affecting the phones performance; but it’s becoming a concern to me and my partner.

Any ideas what might be wrong, and how to fix it?