Click on the More Options icon (three vertical dots), located on the right upper corner of the Record toolbar, and select the feature Merge Parties:
Then, in the Destination section on the right, select the “missing” party, which will be easy to identify as it will display only a name without a Lastname. Next, choose a fully completed party under Source on the left side and copy the Lastname from one party to the other. After doing this, your party will be visible.
NOTE: the Copy/Merge feature is only available to admins and standard users with the Copy/Merge roles. This feature only appears as an option in cases where there is more than one party.