No functions will run with the /function command. I have made a folder in the /datapack folder, called kitpvp, made a valid pack.mcmeta file, made an /data folder with a function under /functions named assemble.mcfunction. When I do /function kitpvp:assemble, I get an error unknown function.
How to reproduce
Install a snapshot server (This may also affect singleplayer, I haven't tested)
Under the /world/datapacks folder, make a /custompack folder
Make a valid pack.mcmeta
Make folders /data/functions and add function.mcfunction with any command
Go into the server and do /function custompack:function

I do not know if I am doing this wrong or if this is a bug. I also don't know if this affects singleplayer or not.