The problem is once your slave has pride set to S you have a Sanity status automaticaly, even with all the rest on S.
In the wiki of the game it is said that the Sanity status is the sum of Nature, Temperament, and Pride Temperament. You cannot have an open slave without being mindbroken anymore, and yes, that's a hugh gameplay problem.
It should play like indicated on the wiki, with the sum of all taken into consideration and apply in the status if the sums of it is D+ or D- to be true.
So either the wiki need to be updated or the description of this trait should change, because those are not compatible.