It's not a bug as such but it can be a finicky process some times. The way to check for this while in layout mode is to move the portal a few pixels. If any fields are no longer "owned" by the portal, they won't move with the portal.
Drag the field completely out side of the portal and release the mouse button. Then drag it back and don't release the mouse button until the edges of the field are completely inside the borders of the portal row. Only use the alignment and position tools found in the inspector to make small adjustments to the field's size and position after it has first been placed in the portal row and you can confirm that the portal now "owns" the field.
The most common way this happens is when you use the inspector tools to move an object that is outside the limits of the portal, tab control, popover, or slider to a position that is inside. That can position the object where it needs to be but the enclosing layout object fails to take ownership of it. That can be useful at times as there are situations where it can be useful to put an object on top of the portal without it being owned by the portal.
Many thanks for this this solved the problem and the rogue field is back as it should be.