Resolution:
For validating SOAP messages, SOAP Envelope XSD as well as the XSDs against which the SOAP Body content is to be validated need to be added to Schemas. In SOAP Envelope XSD, the processContents attribute is set to "lax" for the element "any" inside the type "Body" (see excerpt below).

  <xs:complexType name="Body">
    <xs:sequence>
      <xs:any namespace="##any" minOccurs="0" maxOccurs="unbounded" processContents="lax"/>
    </xs:sequence>
    <xs:anyAttribute namespace="##any" processContents="lax">
      <xs:annotation>
        <xs:documentation>
          Prose in the spec does not specify that attributes are allowed on the Body element
        </xs:documentation>
      </xs:annotation>
    </xs:anyAttribute>
  </xs:complexType>
 
"With this setting, validation will not fail if XSD for validating SOAP Body content cannot be found. If the requirement is for validation to fail if XSD for validating SOAP Body content cannot be found, the attribute processContents needs to be set to 'strict'."

Considerations when configuring validation (Operation Feature 'Validation') of SOAP messages.