Please see 31:17. Ibn Ezra (short) says le-olam always means forever. What about Bemidbar 5:13 where the root means hidden?
When and why did chazal start using it to refer to the world?
The word Ne’elam used in Bamidbar 5:13, which means hidden is not the same as leOlam, which means forever, because the vowels are not the same, therefore the meaning is different. Another example of this would be, the letters Shin, daled, yud, can have two different meanings, depending on the vowels, it can be the name of Hashem, or it can mean my field.