cell formatting disappears

Hello, I’m running LibreOffice v on Linux Mint, Build ID: “1:6.4.7-0ubuntu0.20.04.1”.
I noticed that when I’m performing a border cell formatting (bottom line of cell) and when I’m saving the spreadsheet it reverses the border style to the previous stage. It doesn’t matter whether I format only one cell group (D21:F21) or multiple, independent cell groups in the spreadsheet.
Is this bug already known?

Are you saving as .ods?

Yes, as .ods file.

I cannot replicate, but I have LO Versions of LO Calc prior to 6 had different double-line styles, there is only one now; could it be related to an obsolete double-line style in an older spreadsheet?

The only other suggestion I have is to see if it happens in Safe Mode, If it works OK, it might be corrupted user profile

It;s a single line border style (o.75p). I will test it later under Win10 and will post the results.

I tested it under WIN10 - LO v7.1.2.2, ‘normal’ mode and ‘save’ mode.and in both cases the cell border (bottom, single line, 0.75pt) didn’t work. And then it got more confusing (for me). I marked the neighboring cells (e.g. D21:F22) and set the border for the middle border and voila, it worked. Maybe that’s the way how to do it.
Thanks for your help