The "Haiyou" in Du Fu's poem is not a proper noun. The sea is in the east and this state is in the west, so it is called "Haiyou". This method of calling is not uncommon. Du Fu here It represents only a spatial concept.
Later generations called Dai Jinan, and there was no convention to use this name. Because this poem is about Lixia Pavilion, some people call this pavilion "Haiyou Ancient Pavilion". The source is naturally Du Fu's poem.
Jinan is called "Haiyou"