Thanks for bumping this. On closer inspection, there aren't many ways this could happen and it was fairly easy to replicate. The reservation time was set way too high, looks like someone might have mistaken seconds for milliseconds. This scenario can happen any time an ant gets distracted after it has reserved but before it climbs (like if it entered combat or food gather was disabled), I don't see how having previously harvested a leaf would have caused it - is it possible that it got in a fight and was then out of range of the plant and returned home or was going to harvest but someone nearby dropped a leaf and it decided to pick that up instead? Anyway, reducing the reservation time seems to have fixed it.
Completely unrelated to the construction task though
