Ubuntu 18.04.5 LTS
Libre Office 220.127.116.11
Today I have noticed that footnote lines in my document (100 pages) are on some pages slim and or other pages bold. I reckon that this has nothing to do neither with even/odd page variation nor with number of footnotes. It goes like this: 2xB, 9xS, 3xB, 1xS, 1xB, 2xS, 2xB, 8xS, 2xB, 4xS, 1xB, 7xS, 1xB, 8xS, 1xB, 1xS, 1xB, 6xS, 1xB, 1xS, 1xB, 1xS, 4xB, 1xS, 2xB, 1xS, 1xB, 1xS, 1xB. The footnote lines have identical length but various thickness. I use Zotero footnotes only so there is no Libre Office note (Insert → Footnote/Endnote). I have no experience in DTP and therefore I do not know if it the way it should be. I would prefer to have only one kind of footnote lines - either slim or bold.
The differences are preserved when exported to PDF. It is not a display issue.
Printing it to PDF only changes the pattern to: 5xB, 1xS, 6xB, 1xS, 15xB, 2xS, 4xB, 1xS, 7xB, 1xS, 8xB, 1xS, 2xB, 2xS, 3xB, 1xS, 13xB, 1xS, 1xB.
Thank you for your help in advance and wish you all good health!