When I make a new user, a few days later the user doesn't exist in the system. I don't have any auto restore to backup enabled. Why would a user suddenly disappear?

Some details:

The system has a master and two slave sites in subdomains. We are sharing the users across both subdomains, so we have a third site for users in it's own subdomain, users.oursites.com.

I have SecuritycheckPro running and its Track Action logs list the creation of the new user in one of the slave sites, who also displays after I create them in the User list. The new user is able to login successfully.

Then a few days later, they email me and they cannot login. I check the user list and they do not exist. I remade their account and email them. They can login, and then a few days later, their account does not exist.

By the way, this user's full name has an apostrophe in it, e.g., Ta'Mora. The account creation process was successful, so I don't think Joomla had a problem with the apostrophe in the name.

This is a Joomla 3.8.8 systems with JMS 1.3.73 Full. PHP is 7.1.33.

It happened again. I remade the user account in Joomla in the Users.outsite.com shared user site, it displayed in the slave site user list, then 2 hours later the accountis gone. Also, as a test, I did not include the apostrophe in the users's full name field.

I found the cause. As mentioned above, this multisite shares users. They all have the SecuritycheckPro extension installed, each with its own config settings (separate databases). One of the slaves had a User Session protection parameter enabled prohibiting the creation of new admin accounts. That was deleting the account after it was created on the other slave. So the warning wasn't displayed in that slave admin area. But it was triggered in the slave with the prohibition enabled.