Data API calls from custom plugin do not call the endpoint

I’m trying to add an API call to a custom plugin that’s meant to be used as data source (not action).
When I initialise the call in the plugin editor, it’s retrieving the data correctly.
but when I add an element to a page that’s meant to output the data, the API endpoint never gets called.

Custom plugin config

Data returned by the endpoint

If I setup the exact same call using the API connector plugin it works fine, but for custom plugin, it doesn’t.
API Connector call configuration

Is there some setting setting I’m missing?