Can you create a simple page with the date field and upload control and see if you can make it do the same thing. I'm guessing but I would think that there might be something elsewhere on the page that is getting in the way.  I had something similar the other day and I eventually stripped the page down and isolated the problem then rebuilt the functional page. The solution in my case was pretty simple. I have an axiom 'a bug that is easy to find is hard to fix, a bug that is hard to find is easy to fix---- all bugs take the same time to fix!

Yes, I had to do exactly that, stripped everything out except the date, the upload and a Save button.  I discovered that the document would actually save, and THEN the date would get reformatted.  So, as a workaround, I added code to clear the date field in the oncomplete event of the Save button - and that actually works.  So, not a solution, but a workaround.  (I was able to do that only because the Save performed a partial refresh.  The form actually appears in a dojo dialog panel)

I'm still looking for a solution, because I'm afraid I'll run into this again in the future.

Here's how it looks before Save

and after Save

Here's the xpage code

<?xml version="1.0" encoding="UTF-8"?>
<xp:view xmlns:xp="http://www.ibm.com/xsp/core"><xp:panel id="panelCustPO">
        <xp:this.data>
            <xp:dominoDocument var="document3" formName="CustPOTemp" ignoreRequestParams="true">
            </xp:dominoDocument>
        </xp:this.data>
        
        <xp:table style="background-color:rgb(242,242,242);border-color:rgb(128,128,128);border-style:solid;border-width:thin">
            
            <xp:tr>
                <xp:td style="text-align:right">

                    <xp:label value="P.O. Received Date:" id="label101">
                    </xp:label>
                </xp:td>
                <xp:td style="background-color:rgb(255,255,255)">
                    <xp:inputText id="DateRec" value="#{document3.DateRec}" style="height:20px">
                        <xp:this.converter>
                            <xp:convertDateTime pattern="MMM d, yyyy">
                            </xp:convertDateTime>
                        </xp:this.converter>
                        <xp:dateTimeHelper></xp:dateTimeHelper>
                    </xp:inputText>
                    
                </xp:td>
                <xp:td></xp:td>
            </xp:tr>
            <xp:tr>
                <xp:td style="text-align:right" valign="top">
                    <xp:label value="P.O. Attachment:" id="label225"
                        style="font-size:9pt">
                    </xp:label>
                </xp:td>
                <xp:td style="background-color:rgb(255,255,255)">
                    <xp:fileUpload id="Attachments"
                        value="#{document3.Attachments}" style="width:99%;height:20px">
                    </xp:fileUpload>
                </xp:td>
                <xp:td></xp:td>
            </xp:tr>
            <xp:tr>
                <xp:td colspan="3" style="background-color:rgb(255,255,255);text-align:right;padding-top:10.0px">
                    <xp:button id="button18" value="OK" style="width:80px;margin-bottom:5.0px">
                        <xp:eventHandler event="onclick" submit="true"
                            refreshMode="partial" id="eventHandler55"
                            refreshId="panelCustPO">
                            <xp:this.action>
                                <xp:actionGroup>
                                    <xp:saveDocument var="document3"></xp:saveDocument>
                                    <xp:confirm
                                        message="save successful">
                                    </xp:confirm>
                                </xp:actionGroup>
                            </xp:this.action>
                            <xp:this.onComplete><![CDATA[//var fld = XSP.getElementById("#{id:DateRec}");
//fld.value = ""]]></xp:this.onComplete>
                        </xp:eventHandler>
                    </xp:button>
                </xp:td>
            </xp:tr>
        </xp:table>
    </xp:panel>
</xp:view>