Why project is not imported by QWC Admin GUI? (qwc-services)

I am testing QGIS’s qwc-services docker container stack deployment.
I have cloned the ‘qwc-docker’ GitHub repo and customized the docker-compose.yml to create only the services needed to implement a basic WMS server.
‘Docker-compose up’ goes successful and the deployment works OK with the available qwc-demo QGIS project.

Next is trying to import a simple .qgs project I created with QGIS desktop 3.16 with 1 single layer showing data in a .csv file.
The project works OK in QGIS desktop and I have configured both the map and the layer for the QGIS-Server publishing according to the docs (I think). But my project is not shown in the qwc Config GUI.

I can see that the project is NOT imported into the QGIS server /data path and, looking into few log messages, I discovered that if the layer does not have set a custom property convert_categorized_layer to ‘true’ (in the .qgs xml file) it will be silently skipped and, being the only layer in the project, the whole project is silently skipped.

How can I configure a layer to have such custom property convert_categorized_layer set to true using QGIS-3.16 desktop? And, in general, is there a proven procedure available helping to publish QGIS 3.16 desktop projects into qwc-services stack deployment?