### 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)

float y_off = TS->shaped_text_get_underline_position(rid);
				float underline_width = MAX(1.0, TS->shaped_text_get_underline_thickness(rid) * theme_cache.base_scale);
				draw_dashed_line(dot_ul_start + Vector2(0, y_off), p_ofs + Vector2(off.x, off.y + y_off), dot_ul_color, underline_width, MAX(2.0, underline_width * 2));
			}
			if (_find_strikethrough(it)) {
				if (st_started && font_color != st_color_prev) {
					float y_off = -TS->shaped_text_get_ascent(rid) + TS->shaped_text_get_size(rid).y / 2;
					float underline_width = MAX(1.0, TS->shaped_text_get_underline_thickness(rid) * theme_cache.base_scale);
					draw_line(st_start + Vector2(0, y_off), p_ofs + Vector2(off.x, off.y + y_off), st_color, underline_width);
					st_start = p_ofs + Vector2(off.x, off.y);
					st_color_prev = font_color;
					st_color = font_color;
					st_color.a *= 0.5;
				} else if (!st_started) {
					st_started = true;
					st_start = p_ofs + Vector2(off.x, off.y);
					st_color_prev = font_color;
					st_color = font_color;
					st_color.a *= 0.5;
				}
			} else if (st_started) {
				st_started = false;
				float y_off = -TS->shaped_text_get_ascent(rid) + TS->shaped_text_get_size(rid).y / 2;
				float underline_width = MAX(1.0, TS->shaped_text_get_underline_thickness(rid) * theme_cache.base_scale);
				draw_line(st_start + Vector2(0, y_off), p_ofs + Vector2(off.x, off.y + y_off), st_color, underline_width);
			}
			
			ItemFade *fade = nullptr;
			Item *fade_item = it;
			while (fade_item) {