### 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)

Variant ret = Control::get_drag_data(p_point);
	if (ret != Variant()) {
		return ret;
	}
	if (selection.drag_attempt && selection.enabled) {
		String t = get_selected_text();
		Label *l = memnew(Label);
		l->set_text(t);
		set_drag_preview(l);
		return t;
	}
	return Variant();
}
bool RichTextLabel::_is_click_inside_selection() const {
	if (selection.active && selection.enabled && selection.click_frame && selection.from_frame && selection.to_frame) {
		const Line &l_click = selection.click_frame->lines[selection.click_line];
		const Line &l_from = selection.from_frame->lines[selection.from_line];
		const Line &l_to = selection.to_frame->lines[selection.to_line];
		return (l_click.char_offset + selection.click_char >= l_from.char_offset + selection.from_char) && (l_click.char_offset + selection.click_char <= l_to.char_offset + selection.to_char);
	} else {
		return false;
	}
}
bool RichTextLabel::_search_table(ItemTable *p_table, List<Item *>::Element *p_from, const String &p_string, bool p_reverse_search) {