I made a small test project to illustrate the problem.
This is the form with the popup form shown, when the button is clicked the first time:
[attachment=8696:PopupFrmBug1.jpg]
This is the same form when the button is clicked the second time:
[attachment=8697:PopupFrmBug2.jpg]
As you can see, it looses the visibility of the TsPanel at the bottom. If you look carefully the panel shows initially and then is hided immediately.
This is the test project:
[attachment=8698:acPopupFrmBug.zip]