At the beginning, the trace is in the middle of the screen.
When setting the trace to -5V, the trace is displayed one division lower than the middle of the screen.
When scale is set to 2V, the trace is 2.4 divisions lower than the middle of the screen.
When scale is set to 1V, the trace is at the bottom of the screen.
When scale is set to 500mV, the trace is still displayed at the bottom of the screen. Is this "the bug"?
This is expected, because when the trace is supposed to be out of screen, the oscilloscope still displays the trace as a line at the most upper or lower side of the display area. For example, if you signal is "out of the screen" only some areas, you will not see an interrupted trace. The displaying of a trace is always bounded by the upper or by the lower screen area.
Other said, if a trace should be represented "1 meter under the display", the oscilloscope will still draw a continuous line at the most lower pixels of the display. It is not like on an analogue display where the trace completly dissapear when it is too high or too low. This is not a bug, it was always like that, it was debated before and many people prefer to see a line at the extremes of the display rather than seeing the trace completely disappearing.
To be sure a waveform is correct, never let the trace to touch the most upper or the most lower pixels.