This is my scenario and configuration and it works.
2 Lists
Countries
———–
CountryID
CountryName
Languages
—————
LanguageID
LanguageName
CountryID (this is a lookyp field to the above list firld CountryID)
I have a qListForm set to New Item and list Langauges all 3 fields displayed.
Field CountryID displays first and has Auto PostBack enabled.
The condition for Form Component Behavior is the following (the field LanguageName gets disabled when teh CountryID is equal to CAN, you can add your own values here).
<FormComponentBehavior>
<Component Type="Field" ID="LanguageName">
<DisabledWhen>
<Eq>
<FieldRef Name="CountryID" />
<Value Type="Lookup">CAN</Value>
</Eq>
</DisabledWhen>
</Component>
</FormComponentBehavior>
Everything works as expected.
Please try this scenario and let me know how it goes.