Here is a little script to show the behavior:

clear, close all
graphics_toolkit qt

figure
subplot(2,1,1);
plot(1:5)
legend('legend text at wrong position (too high)');
% if you stop the script here, via a "break" command, then it will be fine
subplot(2,1,2);
plot(1:5)
legend('legend text at proper position');

The legend in the upper subplot is printed at a too high position. Depending on the screen resolution (or fontsize?) this is more or less obvious.

If you stop the script at the marked position (before creating the second subplot), then the legend position will stay correct.

This happens with graphics_toolkit qt as well as fltk. Gnuplot is fine in this respect.

This happens with Octave 4.0.3, and with Octave 4.2.0-rc2, on Linux and on Windows.

I wasn't sure if this is part of the behavior in bug #39697, that's why I opened a new bug report. I naively assume that NO listener would be necessary to fix this issue here, the subplot sizes should be known in advance.