Thank you for your post.
I am unable to replicate the issue. This is what I have done:
1. I created two tables, Master and Portal, with each table containing a Number field named ID.
2. Additional fields were added to both tables, and the tables are linked in the Relationships graph by ID.
3. In the Master layout, I created a Portal into "Portal" showing a couple of fields (P1 and P2).
4. In Browse, the information displays in "Portal".
5. Back to Layout mode, I selected the P2 field in the portal, duplicated it, and changed it to P3. I then placed P3 to the right of P2 (using arrow keys) and still within the portal.
6. In Browse Mode, all three fields display in the portal.
Are you dragging the fields into the portal, or are you using the Position values in the Inspector. If you use the latter, this places the field in a specific location, but it does not necessarily mean it becomes part of the portal. Once you get the position, press the up arrow once and the down arrow once to make sure it now belongs to the portal.
Let me know what I'm doing differently than you so I can replicate the issue.