I have seen a similar problem under https://powerusers.microsoft.com/t5/Building-Flows/Sharepoint-Get-files-properties-only-connector-will-not-return/m-p/247805#M24568 but this did not solve my issue.
When running the Get Files (Properties Only) task, it will return folder data absolutely fine, but no file data.
I have checked the json output, and there are no instances of "{IsFolder}" = False. I have also checked that the Get File Content task works correctly - it does return the file contents.
Clearly I am able to view / read the files, so why is Get Files (Properties Only) not listing them?