Why deleting system files on a rooted Android says "device or resource is busy"? I have FX Explorer, I was trying to delete some stuff in /system/app (the stuff are just empty folders) but it keep saying The operation isn't permitted on a mounted filesystem. What should I do to fix this?

(This issue is new, I used to delete whatever I want without problems)

I tried with the terminal, it says device or resource is busy.
If you are using Magisk modules, you must be trying to delete a bind mounted (1) directory. Enable Core Only Mode (no modules) in Magisk app and reboot. Then delete the stuff you want to.