If you wish to edit this problem out via a mod if you don't play with the spell of mastery victory
<AIRelationsWeight InternalName="ForgeOfTheOverlord">
<Description>Forge of the Overlord!</Description>
<Icon>Event_MeetFaction.png</Icon>
<Weight>-9.0</Weight>
</AIRelationsWeight>
Change this in the CoreAIDefs.xml
If there were a prereq allowing one to detect that the forge of the overlord victory was enabled, I'd be able to produce this effect without modding the core files.