SLD isn't validated or saved between sessions when using ogc:Priority option

I'm using the following ogc options for my SLD for city point labels. It doesn't pass the validation test, but does apply the style defined in the SLD. But, after I apply the style, and try to make additional edits to the sld, the sld is changed, so each time I want to make edits I need to copy/paste from a saved version of the working SLD. Also, if I close my project and reopen, my style is no longer applied to the map and my sld is changed.

<ogc:Priority>
<ogc:PropertyName>POPULATION</ogc:PropertyName>
</ogc:Priority>

Is changed to:
<sld:Priority>
<sld:PropertyName>
<ogc:PropertyName/>
</sld:PropertyName>
</sld:Priority>