You’re correct. There’s no returned value from an API Workflow called in this way (scheduled from your page / Bubble app). What are called “API Workflows” in Bubble might be better termed “server-side workflows”.
These perform actions entirely on the Bubble server and not in the page itself and hence can be run when the user is not present, on a schedule, etc. (You know this, of course.)
Also, these workflows can be exposed as publicly available endpoint and serve as a workflow API for your app. The “return data” workflow action is intended for that use and has no application when an “API Workflow” is scheduled within your Bubble app.
However, what is less commonly known is that, once exposed as an endpoint, your app can actually call its own endpoints via the API Connector. This seems like a wonky thing to do (and if “return data” did work with scheduled workflows, you wouldn’t even bother to do this), but it’s useful and essential in certain cases.
I do not have a “simple” example of this to share, but I talk about this use case and show how its used at the 40 minute mark in the video linked below. You can use this technique to run an API Workflow in a way that returns a value you could use in your page / know when the workflow has completed.
(Obviously, this is not that useful if what you’re doing is scheduling an API Workflow on a list. There are other workarounds for that case, such as writing some data object and checking for its existence.)
Check out the 40 minute mark in the video below for an example: