Robert,
We will be supporting Win 8.1 and .NET 4.5.1 in PTC Mathcad Prime 3.1. However, the vertical broken line issue you see is an issue with .NET 4.5.1 itself. We're in discussions with Microsoft to determine what they can do to fix the issue. We're not yet sure what form the Microsoft solution will take or if when it will be delivered.
Regards,
Andrew McGough