Meshmixer does not split complex of face group, - Red line appears

[hassanaljafar]

When ever I go to split the complex, for some reason this doesn't split this part of the mesh and it gives me a red-line. When ever I go to click and look through to see the selected face group/part of the mesh that has been split in the object browser, I don't this part of the mesh being highlighted. Does anyone know why this is happening?