Zone 1D2 has only 3/20 plots still for sale but its a corner zone on the map, the adjacent zones haven't been generated. I thought it was when 16 had sold in a zone it triggered the adjacent zones to populate? I did a quick walk round, no other edge zone has anywhere like so few available plots. Another little bug I noticed is that while you float the mouse over a sold property it correctly says 'not for sale', but if you click it then it says 'great choice' albeit with no price or purchase button.