### 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)

ClassDB::bind_method(D_METHOD("is_hint_underlined"), &RichTextLabel::is_hint_underlined);
	ClassDB::bind_method(D_METHOD("set_scroll_active", "active"), &RichTextLabel::set_scroll_active);
	ClassDB::bind_method(D_METHOD("is_scroll_active"), &RichTextLabel::is_scroll_active);
	ClassDB::bind_method(D_METHOD("set_scroll_follow", "follow"), &RichTextLabel::set_scroll_follow);
	ClassDB::bind_method(D_METHOD("is_scroll_following"), &RichTextLabel::is_scroll_following);
	ClassDB::bind_method(D_METHOD("get_v_scroll_bar"), &RichTextLabel::get_v_scroll_bar);
	ClassDB::bind_method(D_METHOD("scroll_to_line", "line"), &RichTextLabel::scroll_to_line);
	ClassDB::bind_method(D_METHOD("scroll_to_paragraph", "paragraph"), &RichTextLabel::scroll_to_paragraph);
	ClassDB::bind_method(D_METHOD("scroll_to_selection"), &RichTextLabel::scroll_to_selection);
	ClassDB::bind_method(D_METHOD("set_tab_size", "spaces"), &RichTextLabel::set_tab_size);
	ClassDB::bind_method(D_METHOD("get_tab_size"), &RichTextLabel::get_tab_size);
	ClassDB::bind_method(D_METHOD("set_fit_content", "enabled"), &RichTextLabel::set_fit_content);
	ClassDB::bind_method(D_METHOD("is_fit_content_enabled"), &RichTextLabel::is_fit_content_enabled);
	ClassDB::bind_method(D_METHOD("set_selection_enabled", "enabled"), &RichTextLabel::set_selection_enabled);
	ClassDB::bind_method(D_METHOD("is_selection_enabled"), &RichTextLabel::is_selection_enabled);
	ClassDB::bind_method(D_METHOD("set_context_menu_enabled", "enabled"), &RichTextLabel::set_context_menu_enabled);
	ClassDB::bind_method(D_METHOD("is_context_menu_enabled"), &RichTextLabel::is_context_menu_enabled);
	ClassDB::bind_method(D_METHOD("set_shortcut_keys_enabled", "enabled"), &RichTextLabel::set_shortcut_keys_enabled);
	ClassDB::bind_method(D_METHOD("is_shortcut_keys_enabled"), &RichTextLabel::is_shortcut_keys_enabled);
	ClassDB::bind_method(D_METHOD("set_deselect_on_focus_loss_enabled", "enable"), &RichTextLabel::set_deselect_on_focus_loss_enabled);
	ClassDB::bind_method(D_METHOD("is_deselect_on_focus_loss_enabled"), &RichTextLabel::is_deselect_on_focus_loss_enabled);
	ClassDB::bind_method(D_METHOD("set_drag_and_drop_selection_enabled", "enable"), &RichTextLabel::set_drag_and_drop_selection_enabled);
	ClassDB::bind_method(D_METHOD("is_drag_and_drop_selection_enabled"), &RichTextLabel::is_drag_and_drop_selection_enabled);
	ClassDB::bind_method(D_METHOD("get_selection_from"), &RichTextLabel::get_selection_from);
	ClassDB::bind_method(D_METHOD("get_selection_to"), &RichTextLabel::get_selection_to);
	ClassDB::bind_method(D_METHOD("select_all"), &RichTextLabel::select_all);
	ClassDB::bind_method(D_METHOD("get_selected_text"), &RichTextLabel::get_selected_text);
	ClassDB::bind_method(D_METHOD("deselect"), &RichTextLabel::deselect);
	ClassDB::bind_method(D_METHOD("parse_bbcode", "bbcode"), &RichTextLabel::parse_bbcode);
	ClassDB::bind_method(D_METHOD("append_text", "bbcode"), &RichTextLabel::append_text);