Created attachment 31022 [details] Sample to show this bug. After using RegionUtil.setBorderBottom(), when click in the merged cell, the cell will turn to black, and could see the content. Please run attachment, then click in "AAA" cell and "BBB" cell to check the difference.
There is now a verifying testcase in class TestUnfixedBugs which allows to reproduce this locally.
Created attachment 33069 [details] Save workbook to poi.test.xssf.output.dir attachment 31022 [details] saves the file to "C:/test.xlsx". This fails on my Linux computer. The attached patch saves the workbook to SystemProperty(poi.test.xssf.output.dir)/bug55752.xlsx.
For reference, attachment 31022 [details] committed in r1676965 <https://svn.apache.org/viewvc?view=revision&revision=1676965>.
This is similar to bug 56548 where the actual cause is discussed as having different default-values for foreground/background fill which POI does currently not implement correctly...