When the server's op becomes non-operator using the deop command, all players get a message saying "You are not white-listed on this server!" and are kicked from the server

used mod:
carpet-fixes-1.18.1-1.5.1+v220107
fabric-api-0.46.4+1.18
fabric-carpet-1.18.1-1.4.57+v220119
lazydfu-0.1.2
lithium-fabric-mc1.18.1-0.7.7

There were no rules enabled on the server. It was just the default

What rules do you have enabled on the server?

Then it's probably not caused by carpet-fixes. This might honestly be a vanilla bug

Finding the cause, the 'toggleEnforceWhitelist' rule was the cause
It was resolved when the default value true was changed to false

That rule is by default set to the server.properties. So make sure EnforceWhitelist is set to false in your server properties.

Yes, the value of enforce-whitelist is false
But the default value for toggleEnforceWhitelist was true