relative links get full path prepended

I have some content with relative links. As this is an old site that has been transformed a couple times, I use a plugin to put `.html` on my page slugs. It works great (maintaining old site structure), and static-html-output plugin works fine with that.
However, for the existing relative links, the plugin transforms the link incorrectly.
The content contains a link like
`E to H`
and the static page output has
`E to H`

I'm thinking that part of the code doesn't take into account that the current address might not be a folder, but a HTML file.
Where to look for this in the code?