I found a solution to this problem. All the apps were also showing 0b of data / storage usage, the reason why it happen is because security app didn't have the correct permissions for the usage access.Hi! Mi11 ultra here. I cannot clear the data of any of the apps, the option is greyed out. Doesn't matter if it's a system app or the app downloaded from Google play store.
Do you know any solution? Thank you in advance.
Go to Settings - privacy protection - slide to protection - special permissions in the bottom - usage access - find security app and turn on "permit usage access"
The function of cleaning the data of each app will come back and everything will work properly.