Unfortunately, if this can be fixed by modifying a single value or using a different channel, then it is not a bug with the engine, but a problem with your set up. This has to be narrowed down to being able to reproduce using simple content.
If you can reproduce this using materials within the Starter Content then we can report the bug. It comes down to the complexity of your material, how you are applying that material to other functions and features within the engine, and how those interact with one another.
The 4.5.1 engine version is quite different from the 4.12 release as there were a lot of changes made to foliage and LOD’s between the two versions. In red gnash’s post, states,
“I was able to test this in 4.7.5 with the FPS degradation, but when bring the same project into our internal build of 4.8 I was not able to see the same loss in FPS. In your material, I removed everything but the world displacement pin. Everything else was irrelevant to this issue as the slow down was caused specifically in this part of the material. When this is disconnected and PN or FN tessellation is enabled there is no slow down.”
This means his issue was resolved with a later version of the engine.
The compilation of the three parts you mention foliage + tessellation + LOD using your specific material set up causing a drop in FPS therefore cannot be deemed as a bug. If the issue is resolved by changing a value or pulling from a different channel, there could be an error with the math when manipulating vertices. This is something you would need to investigate in order to determine why pulling from the alpha and using a negative value causes the issue to occur.
Thank you,