Hi!
The PostProcessor is not meant to be used with Paralell Import because it is not thread safe. It also seems like the PostProcessor is being called. The likely reason why the flag isn't changed is because AssetGraphStaticFlagTest (which calls SetFlag) is called from a different thread, so they don't share the same state.

Relevant documentation:
Because parallel imports operate on separate processes, any side effects of your scripted importer code also take effect in that process rather than the main Editor process. Side effects that run on separate processes can cause unexpected problems. For example, if you modify a static variable on a C# class during import on a worker thread, that modification is not automatically visible to code running on the Editor’s main thread.
Any code that you write for importing should always be self-contained, deterministic, and shouldn’t change the context it’s running in (for example, it shouldn’t change the Editor settings or create new assets on disk). Following these rules helps your importer code avoid parallel threading issues. It also ensures that your import results are always consistent, and that the Editor always uses cached artifacts instead of unnecessarily reimporting your assets.
source: https://docs.unity3d.com/Manual/ParallelImport.html