System: i7 PC running Linux Mint 17.3 and Lazarus 1.4.4. I found this problem in a larger application that I reduced to isolate the problem.
The attached demo works on GTK2 but not qt.
Run the attached demo in both GTK2 and qt, changing the compilation directive from GTK2 to qt. When the toggle-button is clicked the contents of both the combo-box and the image above the button should change.
Recompile with qt, and only the combo-box contents will change.
The problem also occurred when the application was compiled with a recent trunk version. A compilation error was shown, but the program ran despite the errors, which may be a pointer to the root cause. It presumably also happens in Laz 1.6.
If someone can confirm the above, I will enter it in the bug tracker.