The answer to this is in this post: Python plugin smproxy keyword arguments aren't working as expected In summary, it is best to use name=label for plugins written in Python.
I did find a workaround. If I want to provide a more descriptive label, use
@smproperty.xml('<PropertyGroup label="The center of the object"><Property function="Center" name="Center"/></PropertyGroup>')
This will result in bold text above the entry area for the center position that says “The center of the object”. Technically, this is not what PropertyGroup is meant for, but it helps clarify things when a name does not provide enough of a hint as to what the entry area is for.