The Build Minimap function is only for the World Partition panel in the editor and doesn’t have any relation to the in game minimap.
From the pic it looks like your ocean is not set up right, with 1200 tiles thats 614400.
Use Ocean extents 614400,614400, use Ocean collision extents 307200, 307200 and leave z value as is.
Go to your waterzone and set the zone X & Y extents to 614400, 614400.
The waterzone is related to playable area for water physics, maybe it could affect minimap bounds ??? not sure