I think I have an explanation for this issue. A while back there was a subtle change in patch notes for morgana, saying that the leash range recalculation went from .5 to .25 seconds, meaning the game checks to see if the person/people your shackling are in range ever .25 seconds down from .5, and if the answer is no, it pops the tether.
If you look at the actual leash range on morgana, it's quite short. Coming back to playing morg, I noticed that I key off the audio aspect of the ult, so i chase and that sound sort of builds up, whuuUUUUUUUU-PAAAOW, and I know that I can pull back right at the very very end of that whuuuUUUU type buildup. I hear the popping PAAAOW at the end that signifies stun + second damage tick, like the sound has already been triggered to play, but the actual tether was broken instants before the the 2nd tick of soul shackles, after the 2nd tick's sound effect has already started playing.
I suppose the leash time recalculation change on her ult was done in an effort to make it more consistent, cause I assume the tether could break the exact same way as it does now before the change, but it was just less likely to happen unless you broke range right when the leash range was being calculated. The actual result, I think of this change was effectively lessening the range to the actual range and making the tether easier to break. I noticed everyone was escaping morg ult in pro games after they made this change.
Technically the morg ult ends on a .5 second interval, so I guess the change should matter but maybe it somehow still does. Before it always felt like you could get away with turning away right before the ult ended and still getting the pop, prolly when the enemy was technically out of range.
That's my theory, just a guess.