### 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 RichTextLabel::_validate_line_caches() {
	if (updating.load()) {
		return false;
	}
	validating.store(true);
	if (main->first_invalid_line.load() == (int)main->lines.size()) {
		MutexLock data_lock(data_mutex);
		Rect2 text_rect = _get_text_rect();
		float ctrl_height = get_size().height;
		
		float old_scroll = vscroll->get_value();
		if (main->first_invalid_font_line.load() != (int)main->lines.size()) {
			for (int i = main->first_invalid_font_line.load(); i < (int)main->lines.size(); i++) {
				_update_line_font(main, i, theme_cache.normal_font, theme_cache.normal_font_size);
			}
			main->first_resized_line.store(main->first_invalid_font_line.load());
			main->first_invalid_font_line.store(main->lines.size());
		}
		if (main->first_resized_line.load() == (int)main->lines.size()) {
			vscroll->set_value(old_scroll);
			validating.store(false);
			if (!scroll_visible) {
				vscroll->hide();
			}
			return true;
		}
		
		int fi = main->first_resized_line.load();
		float total_height = (fi == 0) ? 0 : _calculate_line_vertical_offset(main->lines[fi - 1]);
		for (int i = fi; i < (int)main->lines.size(); i++) {