Is there an error in get_the_tag_list()?

There is a possibility, that this string has been already translated and WP is displaying the translation, which has another format.

Try to pass a comma without a function, or take look at _x, where you can specify the context, you are translating.

EDIT

/**
 * Function fixes unknown issue of an unusual space in the tag list
 * @param string $html a piece of html code
 * @ return string
 */
function fix_tags_separator_bug($html){
    $separator = ", ";
    return preg_replace("/(\s){0,},(\s){0,1}/", $separator, $html);
}
add_filter("the_tags", "fix_tags_separator_bug");