The inheritance of the Simeonites was taken from the territory of Judah, because the share for Judah’s descendants was too large for them. So the Simeonites received an inheritance within Judah’s portion.
Jewish Publication Society of America Version (JPS1917) - Joshua 19:9 >Out of the allotment of the children of Judah was the inheritance of the children of Simeon, for the portion of the children of Judah was too much for them; therefore the children of Simeon had inheritance in the midst of their inheritance.