xsi:type on root nodes is not working as expected

[tefra]

<xsd:schema
  xmlns="ST_targetNS"
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  targetNamespace="ST_targetNS">
     <xsd:simpleType name="Test">
          <xsd:restriction base="xsd:string">
               <xsd:pattern value="1|2" />
          </xsd:restriction>
     </xsd:simpleType>
</xsd:schema>

@dataclass
class Test:
    """
    :ivar value:
    """
    value: Optional[str] = field(
        default=None,
        metadata=dict(
            pattern=r"1|2"
        )
    )

Valid input

<test
    xmlns="ST_targetNS"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="ST_targetNS ST_targetNS00201m.xsd" xsi:type="Test">2</test>

xsdata serializer

<Test xmlns="" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">2</Test