"ncdata" total and available size decrease concurrently after deleting files

Nextcloud version: 16.0.3
OS version: Ubuntu 18.04

The issue you are facing:

I found that my NextCloud is out of storage and it keeps prompting alert
“Your storage is full, files cannot be updated or synced anymore!”
So I deleted some large size files permanently but the alert still there.

I checked the disk space in my NextCloud server and found that the total size and available size of “ncdata” decrease concurrently after I deleted some files. Just like the image below:

Is it normal for this situation? How can I release the free space?

Doesn’t sound normal to me. What is it you have mounted at /mnt/ncdata?