### 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)

main = memnew(ItemFrame);
	main->index = 0;
	current = main;
	main->lines.resize(1);
	main->lines[0].from = main;
	main->first_invalid_line.store(0);
	main->first_resized_line.store(0);
	main->first_invalid_font_line.store(0);
	current_frame = main;
	vscroll = memnew(VScrollBar);
	add_child(vscroll, false, INTERNAL_MODE_FRONT);
	vscroll->set_drag_node(String(".."));
	vscroll->set_step(1);
	vscroll->set_anchor_and_offset(SIDE_TOP, ANCHOR_BEGIN, 0);
	vscroll->set_anchor_and_offset(SIDE_BOTTOM, ANCHOR_END, 0);
	vscroll->set_anchor_and_offset(SIDE_RIGHT, ANCHOR_END, 0);
	vscroll->connect("value_changed", callable_mp(this, &RichTextLabel::_scroll_changed));
	vscroll->set_step(1);
	vscroll->hide();
	set_text(p_text);
	updating.store(false);
	validating.store(false);
	stop_thread.store(false);
	set_clip_contents(true);
}
RichTextLabel::~RichTextLabel() {
	_stop_thread();
	memdelete(main);
}