Switching back from session across sub domains to session based on sub domains

My site is using session across sub domain (domain => '.domain.com'). I want to change it to session based on sub domains so I changed the config to domain => null. Then I got the problem on users that logged in before. Although they already logged out and logged in again, the session is still across sub domains. It works for users never logged in to the site before. Please help me. Thanks. Btw, when I clear all the cookies on the site manually. It also works.

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