Created attachment 122737 [details]
This is because the tspan's font-size is set to the the tspan and the text element when it should only be set to the tspan.
Xisco Fauli committed a patch related to this issue.
It has been pushed to "master":
SVGIO: tdf#97941: Don't double tspan fontsize
It will be available in 5.2.0.
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.