For What it is Worth: I kept track in my current career and can now say that on July 27,1940 the magnetic detanators work and the torpedoes do not run deeper than set.
This may not be the etched in stone date. I wasn't able to sink a ship every day on patrol you see. So it may be a few days earlier. Even a week or two but this date does fit the discription I read....sometime after the Norway Campaign.
I did manage to sink two ships on the 27th. One with the pistol alone and the other with torpedo set to maximum listed target keel depth. Both worked fine.
...and have always worked afterward. I'm now into October 1940. (this in itself is strange asI play with duds enabled and almost never see one after the first patrol)
In future careers I'll stick with 0 depth and impact only...until this date.