### 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)

set_current_thread_safe_for_nodes(true);
	_process_line_caches();
	updating.store(false);
	call_deferred(SNAME("thread_end"));
}
void RichTextLabel::_thread_end() {
	set_physics_process_internal(false);
	if (!scroll_visible) {
		vscroll->hide();
	}
	if (is_visible_in_tree()) {
		queue_redraw();
	}
}
void RichTextLabel::_stop_thread() {
	if (threaded) {
		stop_thread.store(true);
		if (task != WorkerThreadPool::INVALID_TASK_ID) {
			WorkerThreadPool::get_singleton()->wait_for_task_completion(task);
			task = WorkerThreadPool::INVALID_TASK_ID;
		}
	}
}
int RichTextLabel::get_pending_paragraphs() const {
	int to_line = main->first_invalid_line.load();
	int lines = main->lines.size();
	return lines - to_line;
}
bool RichTextLabel::is_ready() const {