While I agree if you never saw one do it, you might say that's a lot of drop but we have 4 Z4000s at the club, some of them the earlier revision and they most definitely do the same thing and there is nothing wrong with them. It's just an apparent voltage vs loaded voltage difference. They are not getting hot, melting terminals, or other typical failure when seen with an actual problem of high resistance connection causing voltage drop.
Cannot stress this enough, a Z4000 when actuated by the handles- does not and will not maintain exactly the same voltage as the load varies. Most any transformer regardless of brand works the same way- most of them just do not have meters digitally displaying the voltage where it becomes obvious.
Just at home tested different recent Z4000 transformers, when set to 18V a the handle with no load, when adding a 3.2 amp resistive load (a 24V rated 3D printer heated bed resistance PCB) the resulting voltage dropped to 13V both displayed on the display and verified using a Fluke 101 meter (meter is not true RMS).
