As mentioned, the World Census people have been fooling around with numbers and models lately.
Before the change, nations who added chemicals to their water supply, such as synthetic EDTA, poison, or neurostimulants, would receive an inappropriate huge boost to Recreational Drug Use. This was sub-optimal because the ranking is meant to track the fondness of citizens for recreational drugs, not the level of chemicals in their bloodstream.
Because we hadn't separated those two things before, one step in correcting the problem was to flip across nations we had erroneously labelled as "compulsory drug use" onto an equivalent mirror score. This allows us to accurately measure the degree of freedom citizens enjoy with regard to recreational drugs, so that any legislation you pass in the future will move your ranking accordingly.
Unfortunately, the correction meant translating some nations from a very high score (maximum mandatory drug use) to a very low one (maximum restriction on drugs). The worst thing is that the "No Drugs" policy may even appear as a result of this new stat position! But this will disappear as soon as you make even a single pro-drug decision.
This is a messy fix in the short term, and no doubt something of a shock for those that were riding high thanks to an exploit they were probably unaware of, as well as a pleasant surprise to those that find their position is improved.
Change can be shocking sometimes, but we're aiming to keep improving the simulation, and more changes will be around the corner. Keep an eye on the Beta page for more incoming shifts!