Re: StreamingFileSink only writes data to MINIO during savepoint
The StreamingFileSink requires that you have checkpointing enabled. I'm guessing that you don't have checkpointing enabled, since that would explain the behavior you are seeing.
The relevant section of the docs  explains:
Checkpointing needs to be enabled when using the StreamingFileSink. Part files can only be finalized on successful checkpoints. If checkpointing is disabled, part files will forever stay in the in-progress or the pending state, and cannot be safely read by downstream systems.