Sitemap truncates entries ending in "-index"

I have some markdown pages called When I look in the sitemap.xml the urls for these files are not there and are replaced by .../something-.

Looks like the parser is being a bit too aggressive with removing index.html from the url!

Has anyone else noticed this, or is it my configuration?

Any suggestions on getting the correct name into sitemap.xml (other than changing the name of the page so it doesn’t have -index at the end)?