Ubuntu, btrfs: kbfs.block_cache.disk_max_fraction not working.

[DiagonalArg]

I've got a btrfs RAID1 device on Ubuntu 22.04, with only ~49GiB free.

$ sudo btrfs fi show /
Label: none  uuid: <...>
        Total devices 2 FS bytes used 407.45GiB
        devid    1 size 476.92GiB used 428.03GiB path /dev/mapper/luks.root.1
        devid    2 size 476.92GiB used 428.03GiB path /dev/mapper/luks.root.2

kbfs is using 20G for the cache:

$ pwd
/home/user/.local/share/keybase/kbfs_block_cache
$ du -sh .
20G     .

That's a lot more than 10% of the free space. To try to get some of that back, I:

keybase config set -f kbfs.block_cache.disk_max_fraction 0.05

and then restarted keybase, to no effect. Even setting it to 0.01 had no effect. I'm using constrained mode:

{
    "current_user": "xxxxx",
    "kbfs": {
        "block_cache": {
            "disk_max_fraction": 0.05
        },
        "mode": "constrained"
    },
    "mountdir": "/run/user/1000/keybase/kbfs",
    "mountdirdefault": "/run/user/1000/keybase/kbfs",
    "users": {
        "xxxxx": {
            <...>
        }
    }
}