The bugs occured with country names in French in my case, but I guess it can occur in many non English languages. I give just one weird example :
{{#display_map:Côte dIvoire~Côte d'Ivoire}}
This displays two (!) markers on the map
Topic on Extension talk:Maps
Just put the ' symbol rather than the HTML code. The HTML code does not work since it contains a semicolon, which is used to separate locations, hence the two markers.