Found a bug and the cause in the new Borders option in CC 2018. The problem was with merging Shading and Border styles in consecutive paragraphs in a sidebar. One element was not merging with the others, so an unwanted top border rule was being added above the first item below the title. This sidebar was formerly a 1-cell table in the text flow I wanted to convert to using Borders and Shading to get page breaks in a book without compositor manual labor. All the parameters were identical, so that wasn’t the problem.
I also had a paragraph style rule applied to the title to internally shade the first word. (Underlines didn’t work here because the rule had to also shift left to meet an anchored graphic icon.) I first tried removing the anchored graphic and setting up the same paragraph rule to all the elements to see if either was the problem. No go. The anchored graphic wasn’t a problem and the merging of consecutive paragraphs wasn’t messed up because of any added paragraph rules.
The problem turned out to be that I had a specific swatch set up of 25% of the Pantone color (this is a 2-color book). I replaced the swatch with the plain 100% Pantone color and specified a 25% percentage in the Shading panel to get the same color. This fixed it. Ironically, the Mixed Ink color I was using for the actual border was not affected by this separate swatch problem, despite both black and the Pantone color having set percentages in its swatch.
So the problem here was a specific named Shading swatch (“PANTONE 3005 C 25%” in my case) somehow affecting the Border merging function. I realize these combined features must be terribly difficult to program, but this is going to be a headache for the web people whose workflow involves setting up specific percentage swatches for usage. But I guess at least we all can use this workaround for print.
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