### 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)

Item *subitem = static_cast<Item *>(E);
		_fetch_item_fx_stack(subitem, fx_items);
		if (fx_items.size()) {
			set_process_internal(true);
			break;
		}
	}
}
void RichTextLabel::scroll_to_selection() {
	if (selection.active && selection.from_frame && selection.from_line >= 0 && selection.from_line < (int)selection.from_frame->lines.size()) {
		
		float line_offset = selection.from_frame->lines[selection.from_line].offset.y;
		
		for (int i = 0; i < selection.from_frame->lines[selection.from_line].text_buf->get_line_count(); i++) {
			Vector2i range = selection.from_frame->lines[selection.from_line].text_buf->get_line_range(i);
			if (range.x <= selection.from_char && range.y >= selection.from_char) {
				break;
			}
			line_offset += selection.from_frame->lines[selection.from_line].text_buf->get_line_size(i).y + theme_cache.line_separation;
		}
		
		ItemFrame *it = selection.from_frame;
		while (it->parent_frame != nullptr) {
			line_offset += it->parent_frame->lines[it->line].offset.y;
			it = it->parent_frame;
		}
		vscroll->set_value(line_offset);
	}
}
void RichTextLabel::scroll_to_paragraph(int p_paragraph) {