It could be a faulty component, try disconnecting the camera. I had this problem and was racking my brain for the last two months trying to think of what the problem could be; I finally figured it out yesterday when trying the screen on a different 7 (I was testing the other board to see if it was good) and it was in a bootloop as well. I decided to disconnect the camera and try it again; this time IT WORKED. I connected it back to the original phone (without plugging in the camera), restored it and it worked as well. Maybe you’ll get lucky and it will be something as simple and fixable as that, but it could also be any component on the board or on the screen assembly causing it. Good Luck!
Just an update for those who may have a similar issue after a screen repair: My issue turned out to be a defective camera OF ALL THINGS! I never would’ve suspected a bad camera to cause this sort of issue, but after I unplugged it, everything (aside from the camera, obviously) worked 100%. Hindsight is 20/20 and it really should’ve been one of the first things I tried, but again, I never suspected it as the cause.