Firstly, LibreOffice ROCKS (Great Software) Minor bug, "Cell Border Padding" is not preserved or saved when saving worksheets as XLS or XLSX file-format.
It is not saved in those file formats, because those formats do not support cell border padding at all: you can't have padding in Excel, thus its creators didn't provide a way to store it. This is similar to inability to save rich formatting to TXT files, which is also not a bug, but a natural file format limitation. Closing NOTABUG. Thank you!
*** This bug has been marked as a duplicate of bug 140185 ***