LastLoginTime disappear from users in /v2/users endpoint

In short - I use /v2/users endpoint to retrieve all users and store their LastLoginTime in my database.

The assumption I made - if the user has LastLoginTime set to today, then tomorrow and on subsequent days this LastLoginTime will be the same or newer.

However, I noticed that in marginal cases some users, although LastLoginTime had a value yesterday, have no value today (LastLoginTime is empty, like never logged in).

I cannot analyze a specific case because I do not have access other than the above endpoint.

What action or situation must occur for a user data such as LastLoginTime to be cleared? Can an administrator manually delete such data for a specific user?