Stellaris - There is no option to negotiate a change to a specialist subject

[Joelok]

Description
There is no option to negotiate a change to a specialist subject

Game Version
v3.4.3 (9d15)

What version do you use?
Steam

What expansions do you have installed?


Do you have mods enabled?
No

Please explain your issue is in as much detail as possible.
When trying to change my vassal into a specialist type, the option does not appear. I have positive relations and 100 loyalty. They are "equivalent" in technology. The only options are vassal and tributary.

Steps to reproduce the issue.


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[Joelok]

I have all expansions including overlord.

 

[Joelok]

Are you only able to get a specialized subject by diplomacy? Are they not available for empires that were taken under the subjugate vassal cassus belli?