You'll need to provide some more detail. What FMP version? What OS?
I just tried this with FMP 10 and two tabs. The fields on the "back" tab do not show through when I enter preview mode.
FMP 10 with MS Vista operating system 32bit
Hmm, here's some tests that might reveal something:
Enter layout mode and try dragging your tab control to a different spot by clicking on and dragging a blank part of the tab control. Do your fields move with the tab or are they left behind? If they're left behind, they weren't part of the tab control even though they looked like they were.
Try creating a completely new tab control and add one field at a time to it, check the results in preview each time. Do you get the same problem?
The fields did not move with the tab. I then copied all the feilds into the tab layout and it solved the issue. Thanks