You can try two things:
1. Check whether the View>On-Screen View Options>True Line Weight toggle is ON. It probably is and you can turn it off so your lines will be hairline. (But you may want True Line Weights, so you may need option 2.)
2. While in the Schedule Viewpoint, go to the Pens & Colors Dialog and see which Pen Set is highlighted as that is the Pen Set currently active in your Schedule. Switch to another Pen Set.
You could also save a View of your Schedule Viewpoint, which will store the Pen Set used. Then you can generate the Schedule by activating its saved View. That should guarantee it is displayed with the correct Pen Set.
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