gpasswd[29283]: user xxx removed by root from group google-sudoers

Question

I added a user xxx for sFTP. The user uses SSH keys for authentication.

All works well for a period of time and suddenly the login failed. The log shows:

gpasswd[29283]: user xxx removed by root from group google-sudoers

No one removes this user xxx.

May I know what causes it and how to fix it?

I'm not sure if it is because the keys will be automatically deleted by Google Cloud? I tried to search for relevant information but to no avail.

--- Added Info: ---

I'm not using OS login..

Here is how I setup the user:

Generate key:ssh-keygen -t rsa -f ~/.ssh/userxxx -C userxxx
Copy and save the private key: sudo cat ~/.ssh/userxxx
Copy the public key:sudo cat ~/.ssh/userxxx.pub
Add the public key using the cloud console: Cloud Console > VM > Edit > SSH Keys

I tried to add the public key using the console again, it works briefly but it is removed automatically again.