### 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.
if (gui.sub_windows.size() < 2) {
		return;
	}
	if (!gui.sub_windows[gui.sub_windows.size() - 1].window->get_flag(Window::FLAG_ALWAYS_ON_TOP)) {
		int index = gui.sub_windows.size() - 1;
		while (index > 0 && gui.sub_windows[index - 1].window->get_flag(Window::FLAG_ALWAYS_ON_TOP)) {
			--index;
		}
		if (index != (gui.sub_windows.size() - 1)) {
			SubWindow sw = gui.sub_windows[gui.sub_windows.size() - 1];
			gui.sub_windows.remove_at(gui.sub_windows.size() - 1);
			gui.sub_windows.insert(index, sw);
		}
	}
	for (int i = 0; i < gui.sub_windows.size(); i++) {
		RS::get_singleton()->canvas_item_set_draw_index(gui.sub_windows[i].canvas_item, i);
	}
}
void Viewport::_sub_window_register(Window *p_window) {