Very odd...NEW customer mailing address

Hi,

HikaShop is not involved in the login/logout system of Joomla.
All HikaShop does when a user login or logout on your website is that it restores the HikaShop data ( current cart, current wishlist, current zone, current currency, ... ) in the user session.

The fact that you see the new email address in the customer section of HikaShop is merely a by product of the fact that the joomla user email address has been changed by something because HikaShop keeps the synchronization of the email address of the user. That way, when something changes the email address of the joomla user (like for example when a user changes that in his Community Builder profile, the modification is also done in the customer record of HikaShop.
But that doesn't mean at all that HikaShop is involved in the problem.

First, make sure that you can reproduce the problem. Otherwise, you can debug a problem which you can't reproduce...
Then, you'll have to be methodic and disable the user/system plugins of your website one by one and between each, try if you get the same issue or not. That way, you'll be able to find which plugin is doing that (assuming that it comes from a plugin which changes the email address of the user on logout).