Unknown function in Latest Snapshot (1.21 Pre-release 3)

I have a little datapack I wrote to help me terraform my world (fill in cavities etc), when I started a new game in the latest snapshot the functions no longer seem to exist.  I don't get any errors in the game output, and the datapack is shown as enabled in the datapack list command, but no functions appear in the auto-complete list when I type /function and I always get an Unknown function response when I try to run one.  I made a cutdown version - it runs fine in 1.20.6 release, doesn't work in latest snapshot.  What's changed/gone wrong?
Folder structure is:
datapacks\cutdown
  pack.mcmeta
datapacks\cutdown\data\terraform\functions\main
 destroy.mcfunction
pack.mcmeta contents:
{
    "pack": {
        "pack_format": 48,
        "description": "World terraforming"
    }
}
destroy.mcfunction contents:
fill ~-2 ~ ~-2 ~2 ~15 ~2 air destroy
running /datapack list
[System] [CHAT] There are 2 data pack(s) enabled: [vanilla (built-in)], [file/cutdown (world)]
running /function terraform:main/destroy
[System] [CHAT] Unknown function terraform:main/destroy