I tried these use cases, and below is the outcome:
Start the Zeebe DMN Worker, Upload a new file => the scan is not triggered
Start the Zeebe DMN Worker, Upload an already existing file, but with a slight change in the content (like adding a new comment to the decision table) => the scan is not triggered
According to the tests I conducted, the dmn-repo directory is scanned only when the Zeebe DMN Worker starts. It is never scanned again all along the app lifecycle.