### 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)

const_cast<RichTextLabel *>(this)->_validate_line_caches();
	int tg = 0;
	Item *it = main;
	while (it) {
		if (it->type == ITEM_FRAME) {
			ItemFrame *f = static_cast<ItemFrame *>(it);
			for (int i = 0; i < (int)f->lines.size(); i++) {
				MutexLock lock(f->lines[i].text_buf->get_mutex());
				tg += TS->shaped_text_get_glyph_count(f->lines[i].text_buf->get_rid());
			}
		}
		it = _get_next_item(it, true);
	}
	return tg;
}
Size2 RichTextLabel::get_minimum_size() const {
	Size2 sb_min_size = theme_cache.normal_style->get_minimum_size();
	Size2 min_size;
	if (fit_content) {
		min_size.x = get_content_width();
		min_size.y = get_content_height();
	}
	return sb_min_size +
			((autowrap_mode != TextServer::AUTOWRAP_OFF) ? Size2(1, min_size.height) : min_size);
}
void RichTextLabel::_generate_context_menu() {