Enumeration intellisense does not work if value is not empty

If there is defined an enumeration in schema for attribute value , the intellisense should show the dropdown list if I press Ctrl+Space.

And it does nicely in 2 cases:
1) after the attribute name and equal sign, there is only one quote.
example <One Two="
example <One Two="F
example <One Two="Fi

2) the value attribute is empty.
example <One Two=""

But if there is already a character in the attribute value it does not work.
example <One Two="F"
example <One Two="Fi"
I press Ctrl+Space when I am INSIDE the quotes. It should work.
I expect that the dropdown should open like in the case with one quote only.

I send a simple XSD I used:

<xs:schema id="XMLSchema1" xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="One">
<xs:complexType>
<xs:attribute name="Two">
<xs:simpleType>
<xs:restriction base="xs:string">
<xs:enumeration value="Three"/>
<xs:enumeration value="Four"/>
<xs:enumeration value="Five"/>
<xs:enumeration value="Six"/>
</xs:restriction>
</xs:simpleType>
</xs:attribute>
</xs:complexType>
</xs:element>
</xs:schema>