If you call setUnifiedTitleAndToolBarOnMac(true) and then use QToolBar->setToolButtonStyle(Qt::ToolButtonTextUnderIcon) you will get texts displayed below the icons on the Mac.

For texts that never change this works fine. But if you put Undo and Redo actions in the toolbar and use the undo/redo framework, the texts vary, e.g. "Undo" (greyed out) becomes "Undo Delete" (after a deletion), and so on. Now what happens on the Mac is that when the width of the text changes (maybe not the first time, but certainly after two or three changes), the toolbar button disappears---you can get it back by resizing the window (even if that means making the windows smaller!).