### 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)

fbg_index.x = MIN(i, fbg_index.x);
			fbg_index.y = MAX(i, fbg_index.y);
			draw_box = true;
		}
		if (draw_box) {
			Vector<Vector2> sel = TS->shaped_text_get_selection(p_rid, fbg_index.x, fbg_index.y);
			for (int j = 0; j < sel.size(); j++) {
				Vector2 rect_off = line_off + Vector2(sel[j].x - theme_cache.text_highlight_h_padding, -TS->shaped_text_get_ascent(p_rid) - theme_cache.text_highlight_v_padding);
				Vector2 rect_size = Vector2(sel[j].y - sel[j].x + 2 * theme_cache.text_highlight_h_padding, TS->shaped_text_get_size(p_rid).y + 2 * theme_cache.text_highlight_v_padding);
				RenderingServer::get_singleton()->canvas_item_add_rect(p_ci, Rect2(rect_off, rect_size), last_color);
			}
			fbg_index = Vector2i(end, start);
			draw_box = false;
		}
		if (change_color) {
			fbg_index.x = MIN(i, fbg_index.x);
			fbg_index.y = MAX(i, fbg_index.y);
		}
		last_color = color;
	}
	if (last_color.a > 0) {
		Vector<Vector2> sel = TS->shaped_text_get_selection(p_rid, fbg_index.x, end);
		for (int i = 0; i < sel.size(); i++) {
			Vector2 rect_off = line_off + Vector2(sel[i].x - theme_cache.text_highlight_h_padding, -TS->shaped_text_get_ascent(p_rid) - theme_cache.text_highlight_v_padding);
			Vector2 rect_size = Vector2(sel[i].y - sel[i].x + 2 * theme_cache.text_highlight_h_padding, TS->shaped_text_get_size(p_rid).y + 2 * theme_cache.text_highlight_v_padding);
			RenderingServer::get_singleton()->canvas_item_add_rect(p_ci, Rect2(rect_off, rect_size), last_color);
		}
	}
}
Ref<RichTextEffect> RichTextLabel::_get_custom_effect_by_code(String p_bbcode_identifier) {