### 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)

menu = memnew(PopupMenu);
	add_child(menu, false, INTERNAL_MODE_FRONT);
	menu->connect("id_pressed", callable_mp(this, &RichTextLabel::menu_option));
	menu->add_item(RTR("Copy"), MENU_COPY);
	menu->add_item(RTR("Select All"), MENU_SELECT_ALL);
}
void RichTextLabel::_update_context_menu() {
	if (!menu) {
		_generate_context_menu();
	}
	int idx = -1;
#define MENU_ITEM_ACTION_DISABLED(m_menu, m_id, m_action, m_disabled)                                                  \
	idx = m_menu->get_item_index(m_id);                                                                                \
	if (idx >= 0) {                                                                                                    \
		m_menu->set_item_accelerator(idx, shortcut_keys_enabled ? _get_menu_action_accelerator(m_action) : Key::NONE); \
		m_menu->set_item_disabled(idx, m_disabled);                                                                    \
	}
	MENU_ITEM_ACTION_DISABLED(menu, MENU_COPY, "ui_copy", !selection.enabled)
	MENU_ITEM_ACTION_DISABLED(menu, MENU_SELECT_ALL, "ui_text_select_all", !selection.enabled)
#undef MENU_ITEM_ACTION_DISABLED
}
Key RichTextLabel::_get_menu_action_accelerator(const String &p_action) {