I am not sure I understand what you are doing or expecting. You said you set the value of the field to domainaaa.com via an update rule and then in your transaction you save the value and are getting domainaaa.com. That is what I would expect to see.
Are you saying that the page that domainaaa.com points to contains a link to domainbbb.com and the user then navigates to that page?
If so once the user starts navigating the HTML control doesn't store / update the field based on the last page the user was on. The HTML control is essentially a web browser to let them go where you allow (or anywhere). If you need to know where they went you may have to build an OpenUI control to provide a wrapper around an HTML control so you can have the transaction get the currently display hostname / url.
--Bill