If you export to PDF and put zoom to 1600% the space between the double lines can be seen.
The file content.xml said:
style:border-line-width-left="0.02mm 0.04mm 0.02mm, that is 0.08 mm, or nearly 0.23 pt.
Printing 0.08 mm at 300 dpi will leave a 0.94 dot for both lines (and space).
EDIT: Setting line width to 9pt (the maximum) these are the values in content.xml.
EDIT 2: Setting the line width to 6pt and 3pt bring some surprises: the proportions of double and thin-thick medium gap remains the same.
See edited source file (border line styles.odt).
Tested with LibreOffice 22.214.171.124 (x86); OS: Windows 6.1.
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