Why some footnotes lines are slim and others are bold?

Ubuntu 18.04.5 LTS
Libre Office
Zotero 5.0.89

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!

My first guess is some display issue due to the use of fixed arithmetics instead of floating-point (resulting in a 1- or 2-pixel line depending on where it ends up) but you seem to exclude this. The same might happen when exporting to PDF, but “differently” because of the different resolution.

I’m not using Zotero, thus I can’t tell if its footnotes are mapped into Writer footnotes (very likely) or not. Please, edit your question to attach a sample file (max 5 pages) still exhibiting the issue. This will allow for detailed analysis.