You have your root domain that directs traffic to a sub-domain where the actual conversion happens, whether it's an email optin for lead generation or an ecommerce sales conversion, you need to make sure the proper marketing channel is being attributed for the conversion.
If you don't set up cross-domain conversion tracking, you'll end up seeing most of your conversions being attributed to your root domain, which isn't accurate. It's only saying this because the visitor clicked on the link from your root domain and then converted on the sub-domain.
In reality the lead probably came from Google Search, Facebook or any number of traffic sources.
To make sure your conversion is being attributed to the correct traffic source you need to set up 'cross domain' conversion tracking in Google Analytics 4.
Fortunately, this is easier than it used to be in Universal Analytics.
You simply need to go into your GA4 Data Stream and add all of your sub-domain URL's like how this video explains:
This FREE 6 Lesson Course Shows You How to Set Up Google Ad Conversion Tracking for Lead Gen Campaigns using Google Tag Manager.