If you resize the tab control so the top or left side of a field falls outside the tab, that's what happens. When I resize a tab, I pay attention to the outline inside the tab that shows the boundary of the fields on the inactive tabs. That helps prevent this problem. You'll have to try it to see what I'm talking about.
You also get weird results with tab controls if even one pixel of your tab control "crosses the line" to become part of your header. If your tab control is close to the header-body boundary, try nudging it down a pixel or two to see if that fixes the problem.
This is bad - it should not be this hard to create and maintain a form with multiple tabs - I have to individually mark each field and its lable to move them and I can not drag them with the mouse I have to inch then along with shift cursor key. Should be able to do this with the mouse. I should also be able to drag a square around the objects I want to move and move them. MS Access can do this why can't Filemaker. If MS Access worked on Mac OS I would buy that and use it. I can build an app hours faster in MS Access then with Filemaker.
Sorry but I find hitting brick walls when using Filemaker very frustrating and in order to use more advanced features I have to pay more money not so with MS Access. Don't see how Filemaker can compete with MS Access.
Scratch that on moving the objects with shift cursor YOU CANT MOVE THEM AS GROUP when in a tab only one by one that is so non productive. This is why it takes hours to develop simple things and it should not.