"++" becomes underline?

I am looking for a good MarkDown editor, and so far haroopad looks very good. I do encounter one "undocumented" feature that is annoying to me: ++some text++ makes "some text" underlined. I do not find this syntax defined in MarkDown, and Haroopad does not seem to provide an option for this behaviour.

I am a C++ developer. So, needless to say, this is especially cumbersome to me.

Can we have a switch to turn off this feature?