This depends on your execId and refreshId.
For what happens during partial refresh, read these blog posts, particularly part three http://www.intec.co.uk/tag/partial-refresh/.
If validation (or conversion) fails server-side, the server-side map of the XPage is not updated because the data is not deemed complete enough for server-side processing.
So the partial refresh skips to Render Response, which posts back what HTML should be displayed to the page. That includes values in fields - you're replacing HTML, so it has to.
If you're save button is refreshing the form area, you'll be replacing the HTML there, so overwriting the values entered by the user with the last valid values.
The recommended approach will depend on your page architecture and what you're saving. One is to move validation to the save() function, by which time the values will have been updated in the DominoDocument (the front-end wrapper for the Document on the server). Another is to only refresh the validation area and, if validation was successful, call context.reloadPage()
or context.redirectToPage()
to effectively skip the partial refresh.