When your PMS hosts checkout on its own domain, GA4 can start a new session when guests return from it, with the checkout domain credited as the traffic source.
To prevent this, in GA4 Admin, open the data stream for your property and add the PMS checkout domain to both the cross-domain settings and the unwanted-referrals list. GA4 then treats the outbound trip to the checkout domain and the return as part of the same session.