I placed one overlay on 2 brush faces and/or over 2 separate brushes at an angle between 0 and 90 degrees. The result is the same, the overlay being split in two with a very noticeable transparent line. What is even stranger, this only happens in-engine but in Hammer the overlay is continuous. I annexed screenshots, in this example the angle is at 90 degrees but as I said it would be the same with any angle, like 30 for instance.