In a game with the Australian Summer Patch Australia conquered the city state of Zanzibar. I liberated the city state later and for some turns, it was as it should be.
Then Sumeria conquered Zanzibar. I had diplomatic visibility with Sumeria and it told me "Sumeria has added the new settlement Zanzibar." So I checked on the map, and Zanzibar was conquered and now a Sumerian city. However, it was still listed in the city state list and it was possible to send envoys to it (and become suzerain again after a shift). As the game progressed, Zanzibar was declared war upon by other civs and city states alike. They never attacked the Sumerian city though, since they were not at war with Sumeria.
So something went wrong when Sumerian conquered Zanzibar.
Then Sumeria conquered Zanzibar. I had diplomatic visibility with Sumeria and it told me "Sumeria has added the new settlement Zanzibar." So I checked on the map, and Zanzibar was conquered and now a Sumerian city. However, it was still listed in the city state list and it was possible to send envoys to it (and become suzerain again after a shift). As the game progressed, Zanzibar was declared war upon by other civs and city states alike. They never attacked the Sumerian city though, since they were not at war with Sumeria.
So something went wrong when Sumerian conquered Zanzibar.