Bright Spot On Inside of Screen

iMac 14,4 : 21.5 inch : mid-2014

Over the weekend I upgraded the HDD in my iMac to an SSD.

This requires removing the screen, which is glued onto the enclosure.

As part of the re-assembly process, I cleaned the old adhesive with 91% isopropyl alcohol, applied new adhesive strips (LeFix, not iFixIt, please forgive me), and put everything back together.

For 3 days, everything was fine.

This morning, there was an irregular spot on the inside of the upper-right corner of the screen. The spot is about 2 1/2 inches / 6 cm long x 2 inches / 5 cm high.

Although it is a bright gray, it is not visible against the dark background of my desktop picture. I only noticed it when I had a white browser window in the corner - because it's impossible to not notice. It's brighter than the picture below makes it appear to be.

The spot had not been there 1/2 hour earlier. I know this because every morning, I log into my parents' security camera system to take a screen shot and record the battery levels of the cameras; because Arlo does not provide any logging functionality. I always have the browser window in the upper-right corner of the screen, to include the clock display in the screen shot.

Does anybody have any idea what could be causing this?

Thanks.