Setup:

You need a second monitor, and set it as an extension display.

Reproduce:

edited: In macOS' display setting, put your second display relatively to the top or bottom or the 4 corners of main display.

Open any simple QT application. Drag the window from your current monitor to the secondary monitor. Call showMaximized() by double clicking the title bar.

Current behaviour: the application gets moved back to the main monitor's workspace and is maximized there.

Expected behaviour: the application should be maximized in the secondary monitor's workspace. All native mac applications behave this way.

Even the QT creator app does this.