Multisite subdomain mapping, user automatically logged into all sub sites at once.

I have a Multisite with Multi-Domain subdomain settings.

My main site is Example.com, I have many subsites which are siteA.exam.com, siteB.exam.com, siteC.exam.com

Some sites are being domain mapped (using the domain mapping plugin) , eg: siteC.com -> siteC.exam.com

Now my problem is:
If I login onto siteA.exam.com, I will automatically be logged into siteB.exam.com. Its the same with sign out.

However I guess it's the way mapped domain works, luckily siteC.com do not have this "automatic login" problem.

I guess it has gotta be a cookie issue where instead different cookies for siteA.exam.com & siteA.exam.com, they share the same cookie called exam.com.

Is there any solution to this? I do not want the sign in to be linked within subdomain installation.

Regards
Bucgene