Changing octaves for ambiti doesn't work correctly

Let either top octave or bottom octave go one up, nothing happens; let it go one down, it drops by two. It seems that applying a value to one of the octaves makes the real value one lower than the input.
It's likely that #274213: Improper behaviour of Ambitus (Octave, Positioning etc.) described this issue, but it needs a better description.
It seems like an inspector (UI) issue, but is actually caused by libmscore code. I will submit PR shortly.