The proof available is taking a 2 minute exposure at around 50 mm. You'll need a 10 or 16 stop ND at around f11. Otherwise, I don't have an image and I'm not buying this lens. See ya.
I tried your scenario on my 24-70 on a D300, without an ND filter (sorry, I've never needed a 10 stop ND filter so I don't have one lying around the shop) , at 50mm; one exposure was 2 minutes at F/11, the other test was 4 minutes at F/10. In addition, when I was at Bryce Canyon recently I ran some 8 minute exposures in the middle focal length ranges around F/5.6. In none of these cases did I notice any "overexposure" problem you speak of.
Since you're unable to provide an example, and haven't given us a complete break down of the shooting situation, there's not a lot any of us can really offer. I'm thinking, off the cuff here because of lack of information/example, that if you're using a filter perhaps you've got an issue with the synergy between the lens and filter more than just the lens itself. But without a more concrete example we're, pardon the expression, shooting in the dark here.
To be serious - while I'm not discounting that you saw what you saw, if it is a "design defect", it's not affecting even remotely the same percentage of the lenses ownership as, say, the FF issues with the original 70-200/2.8 VR lens. No lens is perfect, but your scenario, that I can't duplicate, IS pretty rare and I don't think it's going to be a major stumbling block for most purchasers of this lens (which IMO, while having issues at the wide end, is excellent from 28-70mm and competitive with anything out there in that range).