Square profile skirting

I recently completed a fix for a client using 67x19mm MDF skirting. They are now refusing to pay me because I haven't mitred the internal corners...?

Please correct me if I'm wrong, but I was taught to always mitre the external corners and scribe the internals. With the exception being square skirting, where butt joining the internals is fine!



Any thoughts and feedback on this matter would be much appreciated....