Detach then attach the menu target element before showing the menu once will append the menu to the document body.

[doctorj]

I noticed this issue with DropdownButton but it applies to any element that has DropMenu set.
Sometimes it happens that menu position is at the bottom of the page instead near the button. In that case, menu element is missing top and left style properties, and also does not have dui-dd-* class set.
What I was able to find is that this happens when parent of element that has DropMenu (or the element itself) is removed, and appended again. And one more condition for this to happen is: menu must not be activated before first removal.
It sounds complicated, but it happens all the time when I'm switching views by setting content on AppLayout.
Here is the example and instructions to reproduce:

    public void onModuleLoad() {
        var layout = AppLayout.create("Domino-ui starter");

		Button b1 = Button.create("Button")
						.setDropMenu(Menu.<String>create()
										.setDropDirection(DropDirection.BOTTOM_RIGHT)
										.appendChild(MenuItem.create("Item 1"))
										.appendChild(MenuItem.create("Item 2"))
						)
						;
		DivElement d1 = div().appendChild(b1);

		DivElement p = div();
		p.appendChild(div().addCss(dui_flex, dui_gap_1, dui_m_b_10)
							.appendChild(Button.create("Remove")
											.addClickListener(evt -> d1.remove())
							)
							.appendChild(Button.create("Add")
											.addClickListener(evt -> layout.getContent().appendChild(d1))
							)
		);

		layout.getContent().appendChild(p);
		layout.getContent().appendChild(d1);

        body().appendChild(layout);
    }

There are two buttons, Remove and Add to remove/add DivElement which is parent of Button that has DropMenu set.
First we clik Remove to remove it, then Add to set it back again, and then when we clik on Button. The menu is wrongly positioned at the bottom.

There is a case when this will not happen, so we can start from begining:
First we clik on Button and menu opens nicely positioned. Then we click Remove and Add. After that, we click on Button again, and menu still opens properly.