that isnt strange, that is how it works. 1 star, 4 stars, 7 stars, 9 stars etc are save points. If you have 1 star and you try to upgrade and the upgrade fail, you will lose the 3 stars you tried with, but will keep 1 star. so if you get up to 4 stars (12 blue medals) you are save again from losing stars if your upgrade fails etc.