### 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)

if (r_click_item) {
		*r_click_item = nullptr;
	}
	if (r_click_char != nullptr) {
		*r_click_char = 0;
	}
	if (r_outside != nullptr) {
		*r_outside = true;
	}
	Size2 size = get_size();
	Rect2 text_rect = _get_text_rect();
	int vofs = vscroll->get_value();
	
	int to_line = main->first_invalid_line.load();
	int from_line = _find_first_line(0, to_line, vofs);
	Point2 ofs = text_rect.get_position() + Vector2(0, main->lines[from_line].offset.y - vofs);
	while (ofs.y < size.height && from_line < to_line) {
		MutexLock lock(main->lines[from_line].text_buf->get_mutex());
		_find_click_in_line(p_frame, from_line, ofs, text_rect.size.x, p_click, r_click_frame, r_click_line, r_click_item, r_click_char, false, p_meta);
		ofs.y += main->lines[from_line].text_buf->get_size().y + main->lines[from_line].text_buf->get_line_count() * theme_cache.line_separation;
		if (((r_click_item != nullptr) && ((*r_click_item) != nullptr)) || ((r_click_frame != nullptr) && ((*r_click_frame) != nullptr))) {
			if (r_outside != nullptr) {
				*r_outside = false;
			}
			return;
		}
		from_line++;
	}
}
float RichTextLabel::_find_click_in_line(ItemFrame *p_frame, int p_line, const Vector2 &p_ofs, int p_width, const Point2i &p_click, ItemFrame **r_click_frame, int *r_click_line, Item **r_click_item, int *r_click_char, bool p_table, bool p_meta) {