We have a simple SOS 2.0 test via SOAP, the testing endpoints and request body were saved as the POSTMAN collection which can be imported to see the req/res body:
https://www.getpostman.com/collections/e72b39d08ff37f1e4e0b

This problem was originally produced from
SOS 2.0 Test Suite GetObservation-Test fail for SOAP.
After some detail investigations, this problem might be redirected to the SoapParser which can't handle the inline namespace declaration.

The response body can be validated using other tools without any problem.
But the teamegine reports there is an error for the unrecognized namespace which was defined inline in the soap body, the response is like the following image:
https://github.com/opengeospatial/ets-sos20/blob/master/src/site/resources/img/soap-response-error-desc.png
the xml as well:
https://raw.githubusercontent.com/opengeospatial/ets-sos20/master/src/main/resources/sos-soap-response-demo.xml

The gml namespace was declared in line 6 and again in the om:result (line 26), but in line 28, the namespace was ignored by the SoapParser which causes the error.
In the other hand, the XmlValidatingParser works well without the same problem if there is no Soap wrapper for the response body. So we think it might be the problem from the SoapParser.