DocFx PostProcessor: Adding manifest items results in html files after 2nd run only.

**Operation System**: 'Windows 10'
**DocFX Version Used**: 2.42.3

**Template used**: `default` + 'myCustomTemplate'

**Steps to Reproduce**:
1. implement Manifest IPostProcessor.Process(Manifest manifest, string outputFolder)
2. in Process(...), create some valid .yml file inside same folder DocFx generates .yml files
3. Also add a ManifestItem for each of those files to the manifest.
4. Execute docfx (rebuild in Visual Studio) -> .yml files are created correctly (also those created by my post processor), but html files are not created for .yml files created by my post processor.
5. Execute docfx 2nd time -> html files are created.

It's also strange that html files even exist before calling my post processor!

**Expected Behavior**:
- html files are generated when running docfx 1st time.

**Actual Behavior**:
- html files are generated when running docfx 2nd time.

I tried almost anything I can imagine of. I don't really have any idea left.
I was very happy to get some help to get nested namespaces running (which I could push to github then :-))