I've found the issue and I have it fixed now. Should be OK in the next release.
When using the expression builder, make sure you manually cycle the checkbox that enables the metadata property field. When the expression builder closes, the checkbox, if not already checked, is set checked from code, but fails to set an internal flag needed to include the property when editing the metadata.
The problem is only present in the 64-bit extension. Works fine from a 32-bit application file dialog, e.g. from the PDF-ShellTools Reader open file dialog.
Thanks for reporting.