When you terraform a tile (soil, habitat, terraform), planet class goes up.
But after you load a saved game, planet class is back to original.

Now, I wouldn't be bother with this bug much, if it didn't ruined some of my plans I had for class 5 world, that became class 7 world due to terraforming works. I got farm and everything and start growing.

But after I load the save, it's back to class 5 (although there are still more tiles for buildings), and does not grow anymore.