Form Builder doesn’t allow you to enter an XPath expression for labels (as well as help, hint, and error messages). If you have a case like this where you absolutely need a dynamic label, you’ll need to do this by editing the XForms source from Form Builder. We have an RFE for this, and I added a +1 from you:
https://github.com/orbeon/orbeon-forms/issues/1342Alex
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