Overwriting existing content in Tridion Docs


I'm trying to update the content of an existing Draft status object by uploading a new version of the file to Tridion Docs. We're using SDL Tridion Docs - Content Manager - Build 13.0.4115.1.

Here's my Set-IshDocumentObj command:

Set-IshDocumentObj -LogicalId $objectID -Version $objectVersion -Lng "en-us" -FilePath $fullFilePath -Metadata $ishMetadataFields

That gives no errors, and the metadata fields get updated with the values defined in $ishMetadataFields, but the actual object content does not get updated. I'm fairly sure the settings in $ishMetadataFields are correct, and that $fullFilePath points to the correct file. Have I misunderstood something? Is Set-IshDocumentObj the correct cmdlet to use for overwriting existing content?