How to reproduce:
1. Download attached project and open "Example" Scene
2. Enter Play Mode

Actual result: Several cameras with post-processing effects are rendered and accumulated into a render texture. A final camera blits this texture to the screen. The post effects are not applied to the last accumulated image before being blitted. The frame debugger shows the post process not being called at all. In the attached program, three identical render passes are used to illustrate the issue. The first two render properly, the last does not have the post effects

Reproduced with: 2017.2.2f1, 2017.3.1p3, 2018.1.0b10, 2018.2.0a3