I had a friend who had an issue with a plastic part that was cracking weeks (sometimes months) after manufacture. After a lot of trial and error they discovered it was the release agent they put in the mold that was reacting with something else that was off gassing in the product (this was a CE product). I don’t think that would be the case here, but could be something to try. I suppose it is too late in the process to add a second gate?