### 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)

queue_redraw();
		return true;
	}
}
void RichTextLabel::_process_line_caches() {
	
	if (!is_inside_tree()) {
		return;
	}
	MutexLock data_lock(data_mutex);
	Rect2 text_rect = _get_text_rect();
	float ctrl_height = get_size().height;
	int fi = main->first_invalid_line.load();
	int total_chars = main->lines[fi].char_offset;
	float old_scroll = vscroll->get_value();
	float total_height = 0;
	if (fi != 0) {
		int sr = MIN(main->first_invalid_font_line.load(), main->first_resized_line.load());
		
		for (int i = main->first_invalid_font_line.load(); i < fi; i++) {
			_update_line_font(main, i, theme_cache.normal_font, theme_cache.normal_font_size);
			main->first_invalid_font_line.store(i);
			if (stop_thread.load()) {
				return;
			}
		}
		
		if (sr != 0) {
			total_height = _calculate_line_vertical_offset(main->lines[sr - 1]);
		}