Your border lines have different widths: 0.05 pt (bottom) and 0.25 pt (top). Check Table>Properties.
Your document is converted to PDF as you have designed it. The 0.05 pt double line is probably too thin for your printer and is collapsed to a single line.
On screen, LO displays double lines as pair of lines, even if the zoom factor would required merging them visually into a single line. Writer preserves the “technical” aspect of your line even if this is cheating with the proportions. Use the zoom slider to see than the appearance of the lower double lines do not change until you are at least 200%.
It is a shame there is no real table and cell styles (the present so-called “table styles” are in fact macros which do direct formatting). The difference between your lines comes from a manual formatting discrepancy.
Apart from the printer, my configuration is the same as yours (desktop KDE Plasma).
EDIT 1
I think the documentation is not clear about what the Width parameter does.
After experimenting, it seems that the Width parameter controls the total width of the “brush” used to draw the border. Therefore when using double-pattern lines like Double Thin or Thin Thick, Small Gap, the width does not measure the width of the black line(s) (and with Thin Thick, which one?) but the total “thickness” of the lines and the gap in-between. The pattern is scaled according to the width given. This becomes obvious at high magnification and large “width”.
Unfortunately, there is no data about the percentage of the gap relative to the width.
Your printer is a 600 dpi one. A 0.25 pt line width is roughly 2 pixels. It is hard to divide it into 3 for the outer line, the gap and the inner line. If you really want to use double-lined patterns, you’d better increase the width until the gap is assigned at least one pixel. Otherwise, use a single line.
EDIT 2
@LeroyG’s excellent answer to 281697/borders-with-double-thin-lines-convert-to-single-thin-lines contains detailed technical description on the border properties.
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