### 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)

selection.to_frame = c_frame;
							selection.to_line = c_line;
							selection.to_item = c_item;
							selection.to_char = words[i + 1];
							selection.active = true;
							if (DisplayServer::get_singleton()->has_feature(DisplayServer::FEATURE_CLIPBOARD_PRIMARY)) {
								DisplayServer::get_singleton()->clipboard_set_primary(get_selected_text());
							}
							queue_redraw();
							break;
						}
					}
				}
			} else if (!b->is_pressed()) {
				if (selection.enabled && DisplayServer::get_singleton()->has_feature(DisplayServer::FEATURE_CLIPBOARD_PRIMARY)) {
					DisplayServer::get_singleton()->clipboard_set_primary(get_selected_text());
				}
				selection.click_item = nullptr;
				if (selection.drag_attempt) {
					selection.drag_attempt = false;
					if (_is_click_inside_selection()) {
						selection.from_frame = nullptr;
						selection.from_line = 0;
						selection.from_item = nullptr;
						selection.from_char = 0;
						selection.to_frame = nullptr;
						selection.to_line = 0;
						selection.to_item = nullptr;
						selection.to_char = 0;
						deselect();