Based on the way persistent storage that Docker uses, and the virtual paths it happens that some download agents may have a path that is different from Sonarr’s known paths. This is an unfortunate but understandable dynamic of the mapping.
For instance, in my particular setup I have Deluge running in a Docker container and Sonarr running on the actual host. When Deluge finishes a download it properly notifies Sonarr with the path it knows - however Sonarr is not then able to post process the downloaded item as it cannot access the file - it’s path is different.
It would be helpful to be able to override the completed download path on a per download agent setting to address this. Let Sonarr get the notification from the download agent but prefer the path given in a configuration if it is given.