### 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)

current_line = p_search_previous ? ending_line - 1 : ending_line + 1;
	} else if (p_search_previous) {
		current_line = ending_line;
		ending_line = 0;
	}
	
	while (current_line != ending_line) {
		
		if (current_line < 0) {
			current_line = to_line - 1;
		} else if (current_line >= to_line) {
			current_line = 0;
		}
		if (_search_line(main, current_line, p_string, char_idx, p_search_previous)) {
			scroll_to_selection();
			queue_redraw();
			return true;
		}
		if (current_line != ending_line) {
			p_search_previous ? current_line-- : current_line++;
		}
	}
	if (p_from_selection && selection.active) {
		
		return _search_line(main, current_line, p_string, char_idx, p_search_previous);
	} else {
		return false;
	}
}
String RichTextLabel::_get_line_text(ItemFrame *p_frame, int p_line, Selection p_selection) const {