Several times I've made a mistake with long selections using the poly lasso, I either made an accidental selection/removal that went through an unintended area/volume or I pressed/clicked something that canceled/cleared my prior selection by mistake. IIRC, there was no way to reclaim that selection back and I must start again..
While at it, I don't have zephyr open but recall the edit/selection menu not listing an option that I expected to be present, I think it was "deselect".
While at it, I don't have zephyr open but recall the edit/selection menu not listing an option that I expected to be present, I think it was "deselect".