Chain drops occur on all bikes with single front chainring and dereilleur shifting. Its nothing specific to folders. It is discussed among MTB riders since years and there are several chain guide solutions around. None of which truely works, however. That is why some prefer single speed.
Newer attempt from SRAM to bring 1x11 gear systems to mountainbiking tackle the problem by a combination of higher chainring teeth, plus a "horizontal" shifting derailleur. It is rather expensive though. And there are no big chainrings available yet.