SSDC manager Backup & Restore is validated with Cygwin based OpenSSH.
So as a consequence - SDDC manager supports the Windows sftp as long as it is OpenSSH based.
External sftp is outside VCF. So VCF has little control over the specific implementations of the sftp and their deviations compared to OpenSSH(Cygwin OpenSSH for example). Please work with third party sftp vendor for the resolution.