### 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)

const_cast<RichTextLabel *>(this)->_validate_line_caches();
	if (updating.load()) {
		return false;
	}
	return (main->first_invalid_line.load() == (int)main->lines.size() && main->first_resized_line.load() == (int)main->lines.size() && main->first_invalid_font_line.load() == (int)main->lines.size());
}
bool RichTextLabel::is_updating() const {
	return updating.load() || validating.load();
}
void RichTextLabel::set_threaded(bool p_threaded) {
	if (threaded != p_threaded) {
		_stop_thread();
		threaded = p_threaded;
		queue_redraw();
	}
}
bool RichTextLabel::is_threaded() const {
	return threaded;
}
void RichTextLabel::set_progress_bar_delay(int p_delay_ms) {
	progress_delay = p_delay_ms;
}
int RichTextLabel::get_progress_bar_delay() const {
	return progress_delay;
}
_FORCE_INLINE_ float RichTextLabel::_update_scroll_exceeds(float p_total_height, float p_ctrl_height, float p_width, int p_idx, float p_old_scroll, float p_text_rect_height) {