It's not possible to generate xml without a namespace in a domain to xml mapping. Is there a reason you don't want a namespace? Because I would recommend always using a namespace in xml.
I found an answer in the xsd.
<xs:schema xmlns="urn:iso:std:iso:20022:tech:xsd:pain.008.001.02" xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" targetNamespace="urn:iso:std:iso:20022:tech:xsd:pain.008.001.02">
The parts:
elementFormDefault="qualified"
and
targetNamespace="urn:iso:std:iso:20022:tech:xsd:pain.008.001.02"
declare all elements in the xml are part of the targetNamespace This results in the following statement in the generated xml:
<ns1:Document xmlns:ns1="urn:iso:std:iso:20022:tech:xsd:pain.008.001.02">
Where the part
xmlns:ns1="urn:iso:std:iso:20022:tech:xsd:pain.008.001.02"
states that all elements with ns1: are part of the namespace. "urn:iso......"
In conclusion: the Mendix framework is following the xsd/xml rules and there is no error.
Solved bij google/research, added the answer for future reference.
EDIT: I cannot set my own answer as the best answer, but it is solved.