Visual bug in UE4.26 that only shows up on some GPUs

Hi there.

We recently upgraded to UE4.26 and are now seeing some strange behavior in a foliage shader that only seems to happen on RTX 2080 GPUs. These issues don’t show up with the same code and driver on a GTX 1080.

The problem is that the shader seems to produce non-identical vertex locations in different render passes even though the input data to the shader is the same.

To make the situation stranger if you grab a RenderDoc capture on a machine that experienced the issue you can see the bug when looking at the capture. Sending it to someone else with a GTX 1080 the bug didn’t appear in RenderDoc.

In fact on looking at a RenderDoc capture I recorded using a RTX 2080 I can see a discrepancy in the output position of a vertex when looking at the mesh view vs. debugging the vertex.

Lastly the issue disappears when I disable shader compile optimizations.

Hardware I tested on (that exhibits the issue):
RTX 2080
AMD Ryzen 9 3900X
Windows 10 (10.0.19042)