The SVG files generated with the svg terminal are missing font size units i.e. "pt". This results in incorrect font sizes. Older versions of gnuplot (< 5) do not suffer from this issue.
The following script applied to the generated SVG file can be used to workaround the problem:
sed -i 's/font-size="([0-9.]*)"/font-size="\1pt"/g' file.svg
No, the units are not missing. It's quite perfectly legal and sensible to specify the size without a unit. It's a design choice, not a fault.
Diff:
Duplicate of #2000