### 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)

updating_scroll = true;
	float total_height = p_total_height;
	bool exceeds = p_total_height > p_ctrl_height && scroll_active;
	if (exceeds != scroll_visible) {
		if (exceeds) {
			scroll_visible = true;
			scroll_w = vscroll->get_combined_minimum_size().width;
			vscroll->show();
			vscroll->set_anchor_and_offset(SIDE_LEFT, ANCHOR_END, -scroll_w);
		} else {
			scroll_visible = false;
			scroll_w = 0;
		}
		main->first_resized_line.store(0);
		total_height = 0;
		for (int j = 0; j <= p_idx; j++) {
			total_height = _resize_line(main, j, theme_cache.normal_font, theme_cache.normal_font_size, p_width - scroll_w, total_height);
			main->first_resized_line.store(j);
		}
	}
	vscroll->set_max(total_height);
	vscroll->set_page(p_text_rect_height);
	if (scroll_follow && scroll_following) {
		vscroll->set_value(total_height);
	} else {
		vscroll->set_value(p_old_scroll);
	}
	updating_scroll = false;
	return total_height;
}