We notice that for some accounts in our system, the last_logged_in_date column in the cmn_sec_users table is set to null. We have last_updated_by against some other tables matching with these accounts. So, it appears that these users are logging in, but the last time they did so is not being captured in cmn_sec_users.
The last_logged_in_date is null if the user has never logged into the system. Also check for any potential Gel Scripts that might be touching this column/table, and setting last_logged_in_date to null due to a logic error. The following query can be used to get a list of all scripts that touch the cmn_sec_users table.
lower(script_text) LIKE '%cmn_sec_users%'