** WARNING - THE ACHIEVEMENT DESCRIPTION ON THIS WEBSITE IS INCORRECT. READ BELOW **
It is very easy to get this one confused. On this website the description states "Create (a) Forge Map" - while in reality the achievement is for creating (50) forge maps - at least according to the Xbox App after I unlocked the achievement...
The best method indeed is to place a prop down and then use CTRL + D to replicate the object. If you keep an eye on the object counter you can easily tell when you have 50. Then I went in and "Saved as new" and named it. From there I just kept hitting CTRL + S.
The achievement does not appear to be "bugged" in any way. It is just that people are saving after placing 50 props on a single maps, and are not getting the achievement. Each time you save (CTRL + S) it "creates" a new map, even though it doesn't create a new map file in "Your Files".
Hopefully this helps, and hopefully the achievement description gets updated.