||Response from the organizers:
1. Yes, setting the negative value to zero seems like an acceptable fix.
2. The visible tag is meant to indicate if the feature of interest is visible in a specific image. For example, the flooded road category might have several temporal views of a road, but the visible tag should only be true for the images where the road is actually in a flooded state. Similarly, a construction site sequence might have temporal views of the site after construction is finished, but in those views where the bounding box no longer contains a construction site the visible tag should be false.