The help file for the Tremolo.ms object says the following:
I chose this approach for eighth note tremolos since there is no easy way to hide the flag on eighth notes
Here is a fixed version of that measure. Also, please note that you should mute the notes on each side of the tremolo object if you are using playback.
!NoteWorthyComposerClip(2.751,Single)
|Chord|Dur:8th|Pos:-9,-2|Opts:Stem=Up,Beam=First
|Note|Dur:8th|Pos:-5|Opts:Stem=Up,Beam
|Chord|Dur:8th|Pos:-9,-2|Opts:Stem=Up,Beam
|Note|Dur:8th|Pos:-5|Opts:Stem=Up,Beam=End
|Spacer|Width:200
|RestChord|Dur:16th|Opts:Stem=Down,ArticulationsOnStem,Muted,HideRest|Dur2:4th|Pos2:-7|Color:1
|Spacer|Width:300|Color:1
|User|Tremolo.ms|Pos:-8|Color:1
|RestChord|Dur:16th|Opts:Stem=Down,ArticulationsOnStem,Muted,HideRest|Dur2:4th|Pos2:-2|Color:1
|Spacer|Width:375|Color:1
|Chord|Dur:4th|Pos:-9,-2
!NoteWorthyComposerClip-End