### Godot version

v4.2.rc1.official [ad72de508]

### System information

Godot v4.2.rc1 - Windows 10.0.22621 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1660 Ti (NVIDIA; 31.0.15.3742) - Intel(R) Core(TM) i7-10700F CPU @ 2.90GHz (16 Threads)

### Issue description

While trying to use a custom effect script as a workaround for the [rainbow bbcode effect](https://github.com/godotengine/godot/issues/85031) at the RichTextLabel I found a bug regarding the `CharFXTransform.relative_index`.
The variable is explained as follows in the [documentation](https://docs.godotengine.org/en/4.0/classes/class_charfxtransform.html#class-charfxtransform-property-relative-index): 
```
The character offset of the glyph, relative to the current RichTextEffect custom block.
```
However, when using the script below, the character index shifts one to the left each time instead of being at the same position for each new effect block.
![grafik](https://github.com/godotengine/godot/assets/71942164/7b1b0557-d515-4d56-a25e-d2a2810b2966)


### Steps to reproduce

Add this script as Custom Effect to the RichTextLabel (or see the MRP):
```@tool
extends RichTextEffect
class_name test

var bbcode = "test"

func _process_custom_fx(char_fx):
	if char_fx.relative_index == 5:
		char_fx.color = Color.BLACK
	pass
```
Add this Text to a Rich TextLabel:
```
[test]0123456789[/test]
[test]0123456789[/test]
[test]0123456789[/test]
[test]0123456789[/test]
[test]0123456789[/test]
[test]0123456789[/test]
``` 

### Minimal reproduction project

[relative_index_issue.zip](https://github.com/godotengine/godot/files/13462368/relative_index_issue.zip)

}
				}
				if (!b->is_double_click() && !scroll_updated && !selection.active) {
					Item *c_item = nullptr;
					bool outside = true;
					_find_click(main, b->get_position(), nullptr, nullptr, &c_item, nullptr, &outside, true);
					if (c_item) {
						Variant meta;
						if (!outside && _find_meta(c_item, &meta)) {
							
							emit_signal(SNAME("meta_clicked"), meta);
						}
					}
				}
			}
		}
		if (b->get_button_index() == MouseButton::WHEEL_UP) {
			if (scroll_active) {
				vscroll->set_value(vscroll->get_value() - vscroll->get_page() * b->get_factor() * 0.5 / 8);
			}
		}
		if (b->get_button_index() == MouseButton::WHEEL_DOWN) {
			if (scroll_active) {
				vscroll->set_value(vscroll->get_value() + vscroll->get_page() * b->get_factor() * 0.5 / 8);
			}
		}
		if (b->get_button_index() == MouseButton::RIGHT && context_menu_enabled) {
			_update_context_menu();
			menu->set_position(get_screen_position() + b->get_position());
			menu->reset_size();