Hi Martin,
It is just a that custom namespaces are not supported in section templates, since the idea for section templates is that they are created with Form Builder, and Form Builder doesn't support mapping to a custom XML document, of which support for namespaces would be a sub-feature. For now, I've added the RFE linked below, but I can't guarantee that we'll get to it very quickly.
https://github.com/orbeon/orbeon-forms/issues/2466Alex
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