The plugin didn't account for the fact that submit buttons can have a value which get submitted with the form, not just displayed on the button. To fix this issue I added the small snippet of code below to line 27 of the js file.

value="'+$(this).attr('value')+'"

so the final line ended up being:

$(this).replaceWith('<button id="'+ this.id +'" name="'+ this.name +'" type="'+ this.type +'" class="'+ this.className +'" value="'+$(this).attr('value')+'"><span><span>'+ $(this).attr('value') +'</span></span>');