I've found that my ZT-702S with firmware version 1.03.51 sometimes does not give the correct value for Vmin and Vmax, which then also affects the VPP value. This happens in oscilloscope mode with DC coupling and seems to be when the displayed waveform swings over a large vertical portion of the screen.
The problem is that the Vmin value shows far more negative than it actually is. Vmax can change to a strange value as well and can vary from reading to reading. I haven't seen the problem occur with AC coupling.
Here are some examples, (with the probe in 10X mode):
This is the scope's calibration output at 1.0V/div. Displayed values appear to be correct.

This is the same calibration output at 500mV/div (and the 0V baseline moved down). Vmin has changed to -2.17V and Vmax to 0.99V.

This is the calibration output at 500mV/div with AC coupling. Displayed values appear to be correct.

This is a 5V 25.1kHz square wave at 2.0V/div. Displayed values appear to be correct.

This is the same 5V 25.1kHz square wave at 1.0V/div. Vmin has changed to -4.87V, with the corresponding wrong VPP.

Can anyone confirm this problem? Is it fixed in firmare version 1.03.55 (installed on recent units but not yet available for download)?