As far as I see in our API-Platform implementation, there is no way to POST (creation) or PATCH (update) several entities at the same time in the same call, by sending a json array of objects.
Am I missing something? We have a custom DataItem Provider, that may be messing up the default POST collection operation? It isn't clearly described in the generated Swagger/OpenAPI either that you may want to send an array instead of a single object.