Custom Identity Provider force updates user info even if the sync-mode is set to "import"

I am using custom identity provider with updateBrokerUser overrided as:

And sync-mode set to “import”.

But wierdly for a user that has been registered via the broker, after they mannually updated their account information (e.g: first name, last name), their information get reset to sync-with the social provider information upon the next login.

To my understanding this should only happen when the “sync-mode” is set to “force”…
May I have the information about what might be the reason of the force reset?
And what else information should I provide