Hi,
I have worked with Canvas LMS grade passback issues quite a bit and what you are describing sounds like a submission workflow state problem or possibly missing enrollment context in your API calls.
The silent failures with 200 responses usually happen when the assignment submission type does not match what you are posting, or when the workflow state needs to be explicitly set to graded.
I can help you debug this remotely, typically these issues take about an hour to identify once we look at your actual API payloads and the assignment configurations.
Would be happy to jump on a call to get it sorted out.
You can reach out to me on my email here
Colin