Since it appears that your custom tab is only used to attach a different template to a document, what happens if you don't share the custom XML, but only
1. leave it in Normal.dotm or
2. put it in a separate dotm in C:\Users\userid\AppData\Roaming\Microsoft\Word\STARTUP?
Something like
<customUI xmlns="schemas.microsoft.com/office/2006/01/customui" onLoad="RibbonControl.Onload">
<ribbon startFromScratch="false">
<tabs>
<tab id="CustomTab" label="Transcription">
<group id="customGroup" label="Templates">
<dropDown id="attachTemplate" label="Attach template"
getItemCount="RibbonControl.GetItemCount"
getItemLabel="RibbonControl.GetItemLabel"
getSelectedItemIndex="RibbonControl.GetSelectedItemIndex"
onAction="RibbonControl.AttachTemplate" >
</dropDown>
</group>
</tab>
</tabs> </ribbon
</customUI>