If the ball inside sits well, it's not going to leak. Mine sat on a workbench for a week and didn't lose a drop. That said, I believe you, but I'd like to know more about how it happens, because it's not an inherent design flaw......possibly just something else that can be prevented.
I can see them leaking if something gets jammed between the ball and the opening. That could be introduced by a bird. Maybe she ate some biscotti right before grabbing a drink. Unlikely, though, because the opening is small and flush and it wouldn't be easy to get something inside there from below. More likely, the jam would be caused by something inside the bucket. Leaving the top of the bucket open is going to allow dirt, feathers, bugs, and other debris to get in there and settle at the bottom where they will be drawn toward the nipple opening where they can jam and block the ball from settling. Solution: lid.
The bucket being jostled could also result in the ball rocking off the opening. So just hanging a bucket from a rafter with a rope is going to exacerbate this. Solution: secure waterer.
There could be some brands of vertical nipples that are better than others. Maybe we should be happy paying $5 for 10 instead of paying $10 for 50. Solution: splurge for the higher quality nipples.
Also, from what I've seen, people are screwing vertical nipples into thin wall buckets and, most of the time, not using the right size drill bit. You see all the time how people state you can just use a 3/8 or 5/16 bit. 11/32 is not very common. I have a half dozen name brand drill sets and not one of them had an 11/32 bit. I had to pick up a new set specifically to get one.
If you used the wrong size bit, and your nipple is leaking, the water is going to creep down the plastic to the opening and then down to the end of the metal pin. To many, that would look like it's leaking from the opening.