### 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 (E->get()->line == p_paragraph) {
				subitem_to_remove.push_back(E);
			} else {
				break;
			}
		}
	}
	bool had_newline = false;
	
	for (int i = subitem_to_remove.size() - 1; i >= 0; i--) {
		List<Item *>::Element *subitem = subitem_to_remove[i];
		had_newline = had_newline || subitem->get()->type == ITEM_NEWLINE;
		_remove_item(subitem->get(), subitem->get()->line, p_paragraph);
	}
	if (!had_newline) {
		current_frame->lines.remove_at(p_paragraph);
	}
	if (current_frame->lines.is_empty()) {
		current_frame->lines.resize(1);
	}
	if (p_paragraph == 0 && current->subitems.size() > 0) {
		main->lines[0].from = main;
	}
	main->first_invalid_line.store(MIN(main->first_invalid_line.load(), p_paragraph));
	main->first_resized_line.store(MIN(main->first_resized_line.load(), p_paragraph));
	main->first_invalid_font_line.store(MIN(main->first_invalid_font_line.load(), p_paragraph));
	queue_redraw();
	return true;
}
void RichTextLabel::push_dropcap(const String &p_string, const Ref<Font> &p_font, int p_size, const Rect2 &p_dropcap_margins, const Color &p_color, int p_ol_size, const Color &p_ol_color) {