### 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 (swap) {
				SWAP(selection.from_frame, selection.to_frame);
				SWAP(selection.from_line, selection.to_line);
				SWAP(selection.from_item, selection.to_item);
				SWAP(selection.from_char, selection.to_char);
			}
			selection.active = true;
			queue_redraw();
		}
		Variant meta;
		ItemMeta *item_meta;
		if (c_item && !outside && _find_meta(c_item, &meta, &item_meta)) {
			if (meta_hovering != item_meta) {
				if (meta_hovering) {
					emit_signal(SNAME("meta_hover_ended"), current_meta);
				}
				meta_hovering = item_meta;
				current_meta = meta;
				emit_signal(SNAME("meta_hover_started"), meta);
			}
		} else if (meta_hovering) {
			meta_hovering = nullptr;
			emit_signal(SNAME("meta_hover_ended"), current_meta);
			current_meta = false;
		}
	}
}
String RichTextLabel::get_tooltip(const Point2 &p_pos) const {