API workflow not copying a list of things

I created two identical workflows.

one is in normal workflow, triggered by “workflow” button.
another is in API workflow, triggered by “api” button and triggers the api workflow with relevent params

They do the exact same: first copy a list of things (not over 100) and next add it to attribute of another object.

The normal workflow is working, but the API workflow is not. What could have gone wrong?