### 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)

}
			return table_offy;
		}
		off.y += TS->shaped_text_get_descent(rid) + theme_cache.line_separation;
	}
	
	if (line_clicked) {
		
		if (r_click_item != nullptr) {
			Item *it = p_frame->lines[p_line].from;
			Item *it_to = (p_line + 1 < (int)p_frame->lines.size()) ? p_frame->lines[p_line + 1].from : nullptr;
			if (char_pos >= 0) {
				*r_click_item = _get_item_at_pos(it, it_to, char_pos);
			} else {
				int stop = text_rect_begin;
				*r_click_item = _find_indentable(it);
				while (*r_click_item) {
					Ref<Font> font = theme_cache.normal_font;
					int font_size = theme_cache.normal_font_size;
					ItemFont *font_it = _find_font(*r_click_item);
					if (font_it) {
						if (font_it->font.is_valid()) {
							font = font_it->font;
						}
						if (font_it->font_size > 0) {
							font_size = font_it->font_size;
						}
					}
					ItemFontSize *font_size_it = _find_font_size(*r_click_item);
					if (font_size_it && font_size_it->font_size > 0) {