I suggest that you turn the line off in the way that worked previously and then draw a separate line across the layout near the top of the 'Trailing Grand Summary' part, which you format to show in the colour you want.
I think you have been playing with the borders around the whole 'Trailing Grand Summary' part, where the 'Top', 'Bottom', etc settings may not have any effect. In previous versions of FileMaker, a text box would ignore the same settings, allowing the developer to only set the width and colour of the line on all four sides, not individually - perhaps this is the same.
Best wishes - Alan Stirling, London UK.
I would expect what you are doing would work, but obviously it isn't. So I would try a different approach. Remove the line from the part and then go into the trailing summary part and manually draw a line just 1 point below the top of that part so that it is at the top of the trailing summary. This may not answer your question, but can give you a solution.
You both had the same idea and I think it's an acceptable one. Much more control, etc.