### 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)

case ITEM_TABLE: {
				offset += 1;
			} break;
			default:
				break;
		}
	}
	return p_item_from;
}
String RichTextLabel::_roman(int p_num, bool p_capitalize) const {
	if (p_num > 3999) {
		return "ERR";
	};
	String s;
	if (p_capitalize) {
		const String roman_M[] = { "", "M", "MM", "MMM" };
		const String roman_C[] = { "", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM" };
		const String roman_X[] = { "", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC" };
		const String roman_I[] = { "", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX" };
		s = roman_M[p_num / 1000] + roman_C[(p_num % 1000) / 100] + roman_X[(p_num % 100) / 10] + roman_I[p_num % 10];
	} else {
		const String roman_M[] = { "", "m", "mm", "mmm" };
		const String roman_C[] = { "", "c", "cc", "ccc", "cd", "d", "dc", "dcc", "dccc", "cm" };
		const String roman_X[] = { "", "x", "xx", "xxx", "xl", "l", "lx", "lxx", "lxxx", "xc" };
		const String roman_I[] = { "", "i", "ii", "iii", "iv", "v", "vi", "vii", "viii", "ix" };
		s = roman_M[p_num / 1000] + roman_C[(p_num % 1000) / 100] + roman_X[(p_num % 100) / 10] + roman_I[p_num % 10];
	}
	return s;
}
String RichTextLabel::_letters(int p_num, bool p_capitalize) const {