### 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.from_item = nullptr;
								selection.from_char = 0;
								selection.to_frame = nullptr;
								selection.to_line = 0;
								selection.to_item = nullptr;
								selection.to_char = 0;
								deselect();
							}
						}
					}
				}
			} else if (b->is_pressed() && b->is_double_click() && selection.enabled) {
				
				ItemFrame *c_frame = nullptr;
				int c_line = 0;
				Item *c_item = nullptr;
				int c_index = 0;
				bool outside;
				selection.drag_attempt = false;
				_find_click(main, b->get_position(), &c_frame, &c_line, &c_item, &c_index, &outside, false);
				if (c_frame) {
					const Line &l = c_frame->lines[c_line];
					MutexLock lock(l.text_buf->get_mutex());
					PackedInt32Array words = TS->shaped_text_get_word_breaks(l.text_buf->get_rid());
					for (int i = 0; i < words.size(); i = i + 2) {
						if (c_index >= words[i] && c_index < words[i + 1]) {
							selection.from_frame = c_frame;
							selection.from_line = c_line;
							selection.from_item = c_item;
							selection.from_char = words[i];