When using a masked material the overdraw displays differently for foliage meshes. To test this I created different examples to look at and test.
On the far right in the attached project I have a simple plane that is duplicated several time and placed in the same position to assure overdraw. Next I merged the planes into a single static mesh actor. Then I created a series of instanced planes that overlapped. Finally, I took the merged static mesh and painted it once using the foliage tool.
In 4.13 all of these examples display exactly the same however, in 4.14 the foliage mesh ignores the masked area when calculating overdraw. I attached a screenshot showing this test.
The foliage is showing different results than the other examples
They display exactly the same in the overdraw visualization mode
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