iMac 10.12.3 3.2 GHz Intel Core i5 NVIDIA GeForce GT 755M 1 GB
JAVA 9.0.4+11 x86_64
Even after starting a new world in snapshot 18w07c, adding the functions folder to saves/snapworld/data, and a successful in-game /reload...
Attempting: /function namespacefolder:functioname
Minecraft is not finding namespace functions, returning "unknown function functionname"
Minecraft is not autofilling namespace functions when hitting tab while typing the /function command into chat.
Possibly related, Minecraft is additionally not allowing the use of capital letters in the namespace folder name, immediately creating an error warning even before finishing typing the folder's name (see screenshot). Changing the folder name so that it does not contain any capital letters eliminates this specific error alert, but the folder continues to be unrecognized after /reload.
The same unrecognized namespace functions folder will work correctly in the official release 1.12.2 (even with a capital letter in the folder's name) by typing /function namespacefolder:functioname.