### 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)

return line.get_height(theme_cache.line_separation);
}
void RichTextLabel::_update_theme_item_cache() {
	Control::_update_theme_item_cache();
	theme_cache.base_scale = get_theme_default_base_scale();
	use_selected_font_color = theme_cache.font_selected_color != Color(0, 0, 0, 0);
}
void RichTextLabel::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_MOUSE_EXIT: {
			if (meta_hovering) {
				meta_hovering = nullptr;
				emit_signal(SNAME("meta_hover_ended"), current_meta);
				current_meta = false;
				queue_redraw();
			}
		} break;
		case NOTIFICATION_RESIZED: {
			_stop_thread();
			main->first_resized_line.store(0); 
			queue_redraw();
		} break;
		case NOTIFICATION_THEME_CHANGED: {
			_stop_thread();
			main->first_invalid_font_line.store(0); 
			queue_redraw();
		} break;
		case NOTIFICATION_ENTER_TREE: {
			_stop_thread();
			if (!text.is_empty()) {