### 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("push_strikethrough"), &RichTextLabel::push_strikethrough);
	ClassDB::bind_method(D_METHOD("push_table", "columns", "inline_align", "align_to_row"), &RichTextLabel::push_table, DEFVAL(INLINE_ALIGNMENT_TOP), DEFVAL(-1));
	ClassDB::bind_method(D_METHOD("push_dropcap", "string", "font", "size", "dropcap_margins", "color", "outline_size", "outline_color"), &RichTextLabel::push_dropcap, DEFVAL(Rect2()), DEFVAL(Color(1, 1, 1)), DEFVAL(0), DEFVAL(Color(0, 0, 0, 0)));
	ClassDB::bind_method(D_METHOD("set_table_column_expand", "column", "expand", "ratio"), &RichTextLabel::set_table_column_expand, DEFVAL(1));
	ClassDB::bind_method(D_METHOD("set_cell_row_background_color", "odd_row_bg", "even_row_bg"), &RichTextLabel::set_cell_row_background_color);
	ClassDB::bind_method(D_METHOD("set_cell_border_color", "color"), &RichTextLabel::set_cell_border_color);
	ClassDB::bind_method(D_METHOD("set_cell_size_override", "min_size", "max_size"), &RichTextLabel::set_cell_size_override);
	ClassDB::bind_method(D_METHOD("set_cell_padding", "padding"), &RichTextLabel::set_cell_padding);
	ClassDB::bind_method(D_METHOD("push_cell"), &RichTextLabel::push_cell);
	ClassDB::bind_method(D_METHOD("push_fgcolor", "fgcolor"), &RichTextLabel::push_fgcolor);
	ClassDB::bind_method(D_METHOD("push_bgcolor", "bgcolor"), &RichTextLabel::push_bgcolor);
	ClassDB::bind_method(D_METHOD("push_customfx", "effect", "env"), &RichTextLabel::push_customfx);
	ClassDB::bind_method(D_METHOD("push_context"), &RichTextLabel::push_context);
	ClassDB::bind_method(D_METHOD("pop_context"), &RichTextLabel::pop_context);
	ClassDB::bind_method(D_METHOD("pop"), &RichTextLabel::pop);
	ClassDB::bind_method(D_METHOD("pop_all"), &RichTextLabel::pop_all);
	ClassDB::bind_method(D_METHOD("clear"), &RichTextLabel::clear);
	ClassDB::bind_method(D_METHOD("set_structured_text_bidi_override", "parser"), &RichTextLabel::set_structured_text_bidi_override);
	ClassDB::bind_method(D_METHOD("get_structured_text_bidi_override"), &RichTextLabel::get_structured_text_bidi_override);
	ClassDB::bind_method(D_METHOD("set_structured_text_bidi_override_options", "args"), &RichTextLabel::set_structured_text_bidi_override_options);
	ClassDB::bind_method(D_METHOD("get_structured_text_bidi_override_options"), &RichTextLabel::get_structured_text_bidi_override_options);
	ClassDB::bind_method(D_METHOD("set_text_direction", "direction"), &RichTextLabel::set_text_direction);
	ClassDB::bind_method(D_METHOD("get_text_direction"), &RichTextLabel::get_text_direction);
	ClassDB::bind_method(D_METHOD("set_language", "language"), &RichTextLabel::set_language);
	ClassDB::bind_method(D_METHOD("get_language"), &RichTextLabel::get_language);
	ClassDB::bind_method(D_METHOD("set_autowrap_mode", "autowrap_mode"), &RichTextLabel::set_autowrap_mode);
	ClassDB::bind_method(D_METHOD("get_autowrap_mode"), &RichTextLabel::get_autowrap_mode);
	ClassDB::bind_method(D_METHOD("set_meta_underline", "enable"), &RichTextLabel::set_meta_underline);
	ClassDB::bind_method(D_METHOD("is_meta_underlined"), &RichTextLabel::is_meta_underlined);
	ClassDB::bind_method(D_METHOD("set_hint_underline", "enable"), &RichTextLabel::set_hint_underline);