SDTing via API, one source work the other does not

Well, it depends on whether you're using an API id/key or a bearer token. If you're using api id/key, then you have to generate a signature based on the verb, endpoint, current time, and payload (if any). If you're not doing that, it might be why jira is getting the 400.

If you're using a bearer token, it's much simpler. However, they're newer and not many have started using them.