Your waveform must be completely on the display, or within the max offset listed in the scope specs.
Thanks for the suggestion!
Done it, please, take a look at the results!
Ch1 (yellow) completely on the screen, 0 V offset, 5V/div.
Ch2 (blue) -12 V offset, 200mV/div.
Specs: max offsets at 1× probe ratio are
+/-2 V from 1 mV/div to 499 mV/div,
+/- 100 V from 500 mV to 100 V/div.
Ch1 is within the accepted maximum offset (no offset) and shows corrupt waveform,
Ch2 is way off the maximum accepted offset and shows the expected waveform.
I am slightly confused.