I would not say you could not buy English stage 1. Some stores...

I would not say you could not buy English stage 1. Some stores still have stock but most have only 2,3,4. Another interesting point stage 1 where they have it showing exp date april 22 where as stage 4 mainly june, stage 3 may, stage 2 may-june. So, yes, it seems like stage 1 is out of line with other products.
Another possible explanation could be (and this is pure speculation on my part) a2 is preparing to launch a new stage 1 product. From long time observations shortage and temporary reduction in any product indicates that new updated product in the pipeline.