df output does not change after deletion of a 41G file

Maybe my understanding of what df does is incorrect?

I am wondering what is the problem here

1. checking usage

$ sudo df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       443G  436G  4,6G  99% /

2. identifying a large file

$ sudo find /var/lib/libvirt -name WS19.qcow2 -exec ls -lh {} \;
-rw------- 1 root kvm 41G  7. Okt 2020  /var/lib/libvirt/images/WS19.qcow2

3. removing that file

$ sudo find /var/lib/libvirt -name WS19.qcow2 -exec rm {} \;

4. checking usage again

$ sudo df -h /
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1       443G  436G  4,6G  99% /

so, the deletion of 41G did not change the 'df -h' output whatsoever. What am I doing wrong? rm should get rid of this without moving it to the Trash first I think (even checked the Trash. The 41G file is not in there).

it seem btrfs filesystem df neither catches the deletion

sudo btrfs fi df /
Data, single: total=432.37GiB, used=427.79GiB

The deleted file is probably kept open by some process. You can use lsof | grep WS19.qcow2 to discover the affected process; killing it will release the consumed space.

Be aware that the most likely candidate is a running virtual machine. After killing it, all its data will be gone forever (as you already deleted its backing file).