I am not sure I understood things correctly but me as feudal conquering tribal lands I could upgrade the tribal holding into a castle/city/temple just fine. Or it is just when you a tribal and become feudal that the bug appears?
Still rather bad bug for those it affects so hopefully fixed anyway next patch.
After you or anyone become feudal, you can no longer upgrade tribal land. If you (or your vassals) aquire new tribal land, the only way for it to be upgraded is if you make a new tribal vassal.
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