### Godot version

4.0.3-stable

### System information

Windows 10

### Issue description

In some projects ViewportTexture remains pink and doesn't render viewport content after upgrading from 4.0.2-stable to 4.0.3-stable. If a pair of files viewport.h/viewport.cpp are downgraded back to what they were in 4.0.2-stable, problem goes away.

This is how it looks in 4.0.3-stable:
![4 0 3_stable_viewport_4 0 3](https://github.com/godotengine/godot/assets/1449921/44c191a9-4b67-4b02-90b6-4ab615f62a73)

And this is how it looks if viewport.h/viewport.cpp are downgraded back to what they were in 4.0.2-stable.
![4 0 3_stable_viewport_4 0 2](https://github.com/godotengine/godot/assets/1449921/f5abc605-f129-4222-9c0e-e9a07621dd9d)



### Steps to reproduce

Unfortunately, I cannot come up wit a simple demo project reproducing the problem. It exists in some projects and doesn't exist in others.


### Minimal reproduction project

Cannot produce a simple project reproducing the problem. In simple projects it works fine.
ERR_FAIL_COND(!is_inside_tree());
	for (int i = 0; i < gui.sub_windows.size(); i++) {
		ERR_FAIL_COND(gui.sub_windows[i].window == p_window);
	}
	if (gui.sub_windows.size() == 0) {
		subwindow_canvas = RS::get_singleton()->canvas_create();
		RS::get_singleton()->viewport_attach_canvas(viewport, subwindow_canvas);
		RS::get_singleton()->viewport_set_canvas_stacking(viewport, subwindow_canvas, SUBWINDOW_CANVAS_LAYER, 0);
	}
	SubWindow sw;
	sw.canvas_item = RS::get_singleton()->canvas_item_create();
	RS::get_singleton()->canvas_item_set_parent(sw.canvas_item, subwindow_canvas);
	sw.window = p_window;
	gui.sub_windows.push_back(sw);
	if (gui.subwindow_drag == SUB_WINDOW_DRAG_DISABLED) {
		if (p_window->get_flag(Window::FLAG_NO_FOCUS)) {
			_sub_window_update_order();
		} else {
			_sub_window_grab_focus(p_window);
		}
	} else {
		int index = _sub_window_find(gui.currently_dragged_subwindow);
		sw = gui.sub_windows[index];
		gui.sub_windows.remove_at(index);
		gui.sub_windows.push_back(sw);
		_sub_window_update_order();
	}
	RenderingServer::get_singleton()->viewport_set_parent_viewport(p_window->viewport, viewport);
}
void Viewport::_sub_window_update(Window *p_window) {