This bug occurs frequently.

Repro case

1. Using the New File or Project Wizard, create a Qt Designer Form Class
2. Add labels, radio buttons, combo boxes, line edits, horizontal spacers and vertical spacers.
3. Lay the components out using Horizontal and Vertical layouts.
4. Break the layouts, reposition widgets and lay them out again.

Expected result

The designer maintains the visibility of the widgets

Actual result

During re-positioning, some widgets disappear from the form. They are not covered by any other widget. They appear in the object browser. If you select them in the object browser, you can see the resize box. When you save the form, exit Creator and restart with the last project open, they are gone forever, even in the object browser. Widgets I noticed in particular affected by this bug are labels and spacers.