Volume disk usage not reported correctly

SSH to instance with flyctl console:

# df -h
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs        2.0G     0  2.0G   0% /dev
/dev/vdb        7.9G  1.3G  6.2G  17% /
shm             2.0G     0  2.0G   0% /dev/shm
tmpfs           2.0G     0  2.0G   0% /sys/fs/cgroup
/dev/vdc        988M  288M  634M  32% /app/apps/redacted/authData
# du -sh authData
286M    authData
# touch authData/XXX
touch: cannot touch 'authData/XXX': No space left on device

Volume has 1GB and is mounted as /dev/vdc, it says it has 32% of space used but app is failing to write to it.