As far as I remember, it use to work that you would give the treaty and both sides would be able to see each others view in the fow.
Now this is "Still" the case but there seems to be a change of some sort that has caused a minor bug.
It is possible, unlike before, to add it to both sides of the table separately. pre 6.1 when you added it to the table the treaty on the other side was removed.
I assume it was recently changed so that you can choose to have a treaty that is one sided, eg: I can see what you see but you cant see what I see, and vice versa giving a more flexible trade value of the treaty.
However it dose not seem work this way, also if you add it on your side of the table they love it; if you add it from there side they hate it.
Now if its not meant to be changed, then its just a bug with it not hiding from the other side and the value the AI sees the treaty as depending on which side you pick it from. Also the tool tip still says "both sides get to see each others fow", not something like "allow X to see your side of the fow"
(Sorry if this is confusing, its late and im tired
)