QNAP maintains a list of installed packages in /etc/config/qpkg.conf file. The actual package for Xeams is installed in  /share/CACHEDEV1_DATA/.qpkg/ folder. I have a feeling that the uninstaller on your QNAP is removing the actual files but is unable to modify the conf. Modify the conf file and deleting the actual folder manually should remove it.

Please note that we are not as familiar with the QNAP environment as QNAP's support. Therefore, I'd recommend contacting them to ensure these steps are correct.