When Odoo handles multiple websites, the default website is defined from the settings, by selecting the website in the drop-down : 

Unfortunately, the default website is used in all email template links/buttons. So if the user updates the setting of a website, this website becomes the default website and the customers will be redirected to this website when they will click on a link to open an online quotation, a task, etc.

Fortunately, you can freeze the the default website by adding this record in the System Parameters :- Click on create;

- Key : web.base.url.freeze

- Value : True

Author: CTH