### 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)

bool lrtl = is_layout_rtl();
	bool trim_chars = (visible_characters >= 0) && (visible_chars_behavior == TextServer::VC_CHARS_AFTER_SHAPING);
	bool trim_glyphs_ltr = (visible_characters >= 0) && ((visible_chars_behavior == TextServer::VC_GLYPHS_LTR) || ((visible_chars_behavior == TextServer::VC_GLYPHS_AUTO) && !lrtl));
	bool trim_glyphs_rtl = (visible_characters >= 0) && ((visible_chars_behavior == TextServer::VC_GLYPHS_RTL) || ((visible_chars_behavior == TextServer::VC_GLYPHS_AUTO) && lrtl));
	int total_glyphs = (trim_glyphs_ltr || trim_glyphs_rtl) ? get_total_glyph_count() : 0;
	int visible_glyphs = total_glyphs * visible_ratio;
	Vector<int> list_index;
	Vector<ItemList *> list_items;
	_find_list(l.from, list_index, list_items);
	String prefix;
	for (int i = 0; i < list_index.size(); i++) {
		if (rtl) {
			prefix = prefix + ".";
		} else {
			prefix = "." + prefix;
		}
		String segment;
		if (list_items[i]->list_type == LIST_DOTS) {
			prefix = list_items[i]->bullet;
			break;
		} else if (list_items[i]->list_type == LIST_NUMBERS) {
			segment = itos(list_index[i]);
			if (is_localizing_numeral_system()) {
				segment = TS->format_number(segment, _find_language(l.from));
			}
		} else if (list_items[i]->list_type == LIST_LETTERS) {
			segment = _letters(list_index[i], list_items[i]->capitalize);
		} else if (list_items[i]->list_type == LIST_ROMAN) {
			segment = _roman(list_index[i], list_items[i]->capitalize);
		}