Based on my testing this morning, I suspect this works when you build your site with hugo, but fails to update upon changes to your site when running hugo server.
When using Hugo Pipes, the documentation states that:
Asset files must be stored in the asset directory. This is
/assetsby default, but can be configured via the configuration file’sassetDirkey.
You, on the other hand, are attempting to use Hugo Pipes in conjunction with Page Resources. This combination is undocumented, and at least one other person has encountered the problem I described above:
Question for @bep (sorry):
Is processing of Page Resources with Hugo Pipes a supported operation when running hugo and hugo server?