The setting to prevent this from happening is "cross-domain tracking

[shaownhasan]

In the case of cross-domain, if you do not set up "cross-domain tracking", the referrer cannot be measured correctly in Google Analytics.
This is because the session will be terminated when crossing domains.

For example, suppose a person comes to the e-commerce site "aaa.jp" from an organic vp hr email lists search, puts a product in their cart, and transitions to a cart on "bbb.com."
In this case, even though the person actually came from an organic search, the session is terminated when crossing domains, and the referrer is overwritten to "aaa.jp."
A session that should be considered as coming from an organic search is considered to be coming from an external site.
In this cross-domain situation, the original referrer cannot be measured correctly.

Cross-domain tracking is a setting required when measuring page transitions across domains. If
the domains of the cart and the main site are different, like the e-commerce site mentioned earlier, by making this setting, you can properly track the user's referral source and page transition status even in the case of page transitions across domains.