You can’t show a melody on an unpitched staff. When you copy and paste a cue of a pitched instrument onto an unpitched instrument, the cue becomes unpitched. If you need to show a pitched cue on an unpitched staff, you’ll need to assign a pitched instrument to the player holding the percussion kit, then add the cue to that instrument, which should (provided instrument changes are allowed) cause the cue to appear by way of an instrument change at the appropriate point. In Engrave mode you can then hide the redundant label or labels that warn you about the switch to and from the pitched instrument.