Product and version: (FileMaker Pro 15.02.220)
OS and version: OS X 10.12.1/ Windows Server 2012 R2 Essentials (tested on both with FileMaker Pro and with FileMaker Server 15 only on Windows)
On an interface file (i.e.: a file that works on tables from another file) you use a portal to create a related record in a table. If you create a new window on this file and try to edit the prompt row on the portal, you'll see a new record being created for each field you edit. E.g.: fields Name and Surname in the portal; you edit the Name field. As you jump to the Surname field, a new blank prompt row is created into the portal and a record with only the name field filled appears into the target table. As you edit the Surname field and jump to the next one, same thing happens. So, instead of having one record with Name and Surname filled, you obtain two records, one with only the Name field filled, and one with only the Surname.
How to replicate
Create a file with a simple table and two or three fields. Then create a new file and add the first file as external data source. Add the first file table into the relational graph. Then you have to set a relationship to create the new record through the portal. You can do it either with an auto-relation, either starting from another table, as you wish. I tested both having the same problem. To set the relation, I used a global numeric calculation set to 1 into the starting TOC, and a flag "New" (numeric) into the target TOC. Obviously check the allow record creation option under the target TOC. Then you can simply create a layout on the target table, better in list view. Add the portal based on the above relation into the heading section. Then open a new window and try to edit the fields into the portal.
Have a look to the attached file: I have already set it up.
Thank you for your attention.