### 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)

total_height = _resize_line(main, i, theme_cache.normal_font, theme_cache.normal_font_size, text_rect.get_size().width - scroll_w, total_height);
			total_height = _update_scroll_exceeds(total_height, ctrl_height, text_rect.get_size().width, i, old_scroll, text_rect.size.height);
			main->first_resized_line.store(i);
		}
		main->first_resized_line.store(main->lines.size());
		if (fit_content) {
			update_minimum_size();
		}
		validating.store(false);
		if (!scroll_visible) {
			vscroll->hide();
		}
		return true;
	}
	validating.store(false);
	stop_thread.store(false);
	if (threaded) {
		updating.store(true);
		loaded.store(true);
		task = WorkerThreadPool::get_singleton()->add_template_task(this, &RichTextLabel::_thread_function, nullptr, true, vformat("RichTextLabelShape:%x", (int64_t)get_instance_id()));
		set_physics_process_internal(true);
		loading_started = OS::get_singleton()->get_ticks_msec();
		return false;
	} else {
		updating.store(true);
		_process_line_caches();
		updating.store(false);
		if (!scroll_visible) {
			vscroll->hide();
		}