This issue is quite specific. If you have multiple scenes that reside in different bundles then post-processing will most often break hard with unknown shaders error (trying to render a material with unassigned/broken shader, invalid pass number (1) for graphics.blit). In my case, I had 2 scenes that both were in different bundles, both using post-processing. After loading the first scene I loaded into the second one. After that loaded back into the first scene and post-processing worked no more during that build session. Packing both of those scenes into one bundle helped. I feel that there may be dependency issues. Can you please clarify on how to mitigate this particular issue (since you can't really snoop around in this department with packages)? Trying to shove the shaders into Resources didn't help.