Hide treble or bass clef staff on lines where it is only rests

Your piano tsaves have their "hide when empty" property (in Staff/Part Properties) set to Instrument - meaning, the staff is only hidden if the entire instrument is empty. And indeed, that's normally correct - classical piano music virtually never hides only one staff of a piano part. It's either both or neither. Only in jazz and other similar styles do we see piano parts reduce to one staff, as it's common to express passages with just chord symbols. But you can override this using the control I mentioned, if you are positive it won't confuse the pianist (like if they normally play jazz).