If I format a cell border with double lines, it does not save. When I re-open the file the cell has no border. If I format with a single line, it works.
Libre Office version 5.2.2.2
See bug tdf#88827 and the bugs listed there as related. The issue should be fixed with V5.2.4 and higher.
From one of my comments added to the mentioned bug report:
The original bug did not show as soon as the border width (for the double line!) was set to a minimum of 1.25 point (default setting was 0.75 point).
The cycle of creating an example, saving it in xlsx, reloading it, saving it in ods again, an reloading it another time leads to the changed border width of 1.75 point.