I did not do anything specific, except to save the Properties mapping, save the diagram file (graphml) and close it.
Then reopen the diagram (graphml) file, and reopen the Properties Mapper dialog.
[Edit] In fact, I do not have to close the Graphml file: Just open the Properties mapper dialog, click on the "Single template" selector button ( [?] ) button, and select a different node. Close the mapper dialog. The graphml shows a * in the tab ("unsaved changes" status). Save it by pressing CTRL+S. The * disappears from the tab.
Now reopen the Properties mapper. Look at the node. It is different.