### 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)

}
			break; 
		}
		int brk_end = p_bbcode.find("]", brk_pos + 1);
		if (brk_end == -1) {
			
			txt += p_bbcode.substr(brk_pos, p_bbcode.length() - brk_pos);
			add_text(txt);
			break;
		}
		String tag = p_bbcode.substr(brk_pos + 1, brk_end - brk_pos - 1);
		Vector<String> split_tag_block = tag.split(" ", false);
		
		String bbcode_name;
		typedef HashMap<String, String> OptionMap;
		OptionMap bbcode_options;
		if (!split_tag_block.is_empty()) {
			bbcode_name = split_tag_block[0];
			for (int i = 1; i < split_tag_block.size(); i++) {
				const String &expr = split_tag_block[i];
				int value_pos = expr.find("=");
				if (value_pos > -1) {
					bbcode_options[expr.substr(0, value_pos)] = expr.substr(value_pos + 1).unquote();
				}
			}
		} else {
			bbcode_name = tag;
		}
		
		String bbcode_value;