Paul, I have found that importing XML files often results in a duplicate [hidden] clef at the start of the system. Selecting the clef and deleting it seems to eliminate the duplicate and leaves the original intact. Then the system will be hidden (if this option is selected in the layout options).
I’ve also found it helpful to reassign the instruments in the Setup to get proper sounds and symbol placement (vocal staves vs. instrumental staves). Also Select ALL and Reset Appearance, Rest Position, Reset Beaming straightens out numerous issues for importing XML files. YMMV.