It was not on Mac sir
Here is a Screenshot of an App @Jeannot was writing in JavaFX and running on MAC:
he showed this Screenshot in another Thread on this Site you may look on it here:
as you can see it is not a bug. It is so that I was running on Linux with a Design near to MacOS and of course it was there the other way around.
Have a nice day
As addendum: it is also possible to place the menu bar on top of the screen, you can decide by self how you want to do this.
In Java Swing and also JavaFX the Window by self is controlled by the OS, Java is not controlling that. You can do that when setting the Window on undecorated and styling the window decoration by self but I would not recommend that.