I sometimes use a similar setup. The end of the extension cord at the female end where the male end of the Zero cable plugs in it gets warm (not burning hot, but almost uncomfortably warm). The cord itself does not get even warm. The wall end of the extension cord and the bike end of the Zero cord are ever so slightly warm but not hot at all.
I think the issue is with sub-optimal contact surfaces between the Zero male plug and the female end of the extension cord. That, or the way that end of the extension cord attaches to the cord itself, causing heating.
If I plug the Zero cord without extension, it gets slightly warm on both ends, more so at the wall side, but not hot at all.
That is with US voltage, 120V, so 12-15A.
I think the main issue is the poor contact points at the plugs. The cord itself is plenty thick, but at the connection points there are usually cord-to-plug connection quality and pin-to-socket less than ideal contact area and tension issues. So best to use good quality plugs and extension cords that minimize the resistance at the plug ends that causes the heating.
I have a 2015 Zero S 12.5 and the charging cord is connected to a 25' 14 AWG extension cord which plugs into a 20 amp outlet. The end of the charging cord that is plugged into the Zero is hot but not too hot to touch. I cannot hold onto the other end of the charging cord as it is very hot. The charging cord is warm in between the ends but the extension cord is not warm. Has anyone modified their charging cord so that it can be plugged into a 220v outlet thereby cutting the amperage in half?