A bug? Or is it that they don't have enough tech to be a specialist?
Even though Corporate Overlords cannot have a Protectorate subject, the 'Protectorate Tech' check is still in effect on Specialist empires. If they have less than 0.5 of your tech, they cannot become a Specialist (though they can remain a specific type of Specialist - we won't force you to change because you fall behind on tech).
Except you have to be Superior in all factors (Military, Economic,
and Technologically) in order to get the Subjugate casus belli... my experience so far is that it's impossible to force someone to be a vassal anymore early enough to actually matter in the long run. It feels really weird to me that military force isn't enough to get the casus belli... isn't that the point of specialist vassals? To fill in the gaps you don't want/can't fill in yourself? It kind of feels like to get vassals by force, you have to be strong enough to not need them in the first place. I can have the military force to stomp my way across your systems and devastate you into submission at any time.... but because my tech is roughly the same as yours, or my economy is roughly even, I somehow
can't force you to be my vassal. Regardless of how many ships bombard your homeworld.
If the tech requirement for making a specialist is 0.5, yet you require more than that just to declare a subjegation war, then you flat out cannot subjegate someone into becoming a specialist... which makes sense I guess, but now you have to get so far ahead of your target that you
can make them a vassal, then you have to conquer them, wait till you can modify the vassal contract, then make them a specialist.... which by then might not be long enough to actually really take advantage of the specialist system in the first place.
I really feel like requiring tech and economy to be superior in order to get the subjegation casus belli is too harsh right now. By the time you meet those requirements, you probably don't need vassals anyway.