This is a fairly common problem specific to MusicXML imports. The first note of each flow will have the “Starts Voice” property turned on, and the last note in the flow will have the “Ends Voice” property turned on. The solution is to find the last note or rest before the bar rests disappear, select it and turn off the “Ends Voice” property. Then find the first note/rest after a period of no rests, select it and turn off its “Starts Voice” property.