In FileMaker Pro Advanced 184.108.40.2062 and earlier versions of 16, if I re-size the tab object, I have had Portals on the current front tab loose connection to the tab. It still appears to be placed in the selected tab until you change tabs. The object stays in front and appears in front of items on the newly selected tab. The problem can also occur when adding a new portal to a Tab.
The solution is put the Portal where it belongs. Change tabs to see if it is connected. If not, return to the correct tab. Select the Portal and using the keyboard arrow keys move it back and forth or up and down a couple of pixels then put it in the desired location.
Thanks for looking into this.