The topic is self-explanatory. The POG editor doesn't work with D1 levels. The cause is probably that the only six PIG files that it recognizes are all D2's. Might I suggest a manual PIG choice menu instead of a dropdown list? That would solve the issue and also let us use custom palettes such as Pumo's new one that brought the D1 palette to D2.
It's kind of an unintended side-effect - it wasn't intended to work with D1 levels because D1 doesn't support POGs. However it has been brought to my attention that some versions of D1 work with .DTX files. I need to investigate what happens in D2X-XL, and between these will figure out how to deal with D1 in a way that makes sense.