Bubble’s list functions don’t work so well with converting a list of objects to JSON.
You’re better off having no parameters. When making the call, construct the JSON body as a text including the list objects.
If you’re collecting the data on client-side, from database things, you can build the JSON with Toolbox List Item Expression.
If you’re collecting the data from javascript, you can get it back to Bubble with Javascript To Bubble.
If you’re makign the API call on a API workflow, you can use Server Script.
Examples linked to here: