I opened a perfectly square JPG (4000 by 4000) which was created to be a seamless tile.
I used Ctrl T and set it at 50%, constrain proportions, and selected a corner for the reference point location.
After using Ctrl J three times, I moved the layers so they would be like four tiles touching. I used the move tool and the shift key to help them align perfectly.
When they merge (and before merging), the edges are not sharp edged pixels, but slightly transparent pixels (feathered, I guess), so a light line appear across the middle vertically and horizontally.
I also tried duplicating the bottom layer several ways before transforming and had the same problem with feathering/transparency.
I am using Photoshop Elements 2018, and assume this might be the same in CC.
I hope someone knows about this and has a solution.
Thanks for your help!Kathy Keith