### 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)

accept_event();
			}
		}
	}
	Ref<InputEventMouseMotion> m = p_event;
	if (m.is_valid()) {
		ItemFrame *c_frame = nullptr;
		int c_line = 0;
		Item *c_item = nullptr;
		int c_index = 0;
		bool outside;
		_find_click(main, m->get_position(), &c_frame, &c_line, &c_item, &c_index, &outside, false);
		if (selection.click_item && c_item) {
			selection.from_frame = selection.click_frame;
			selection.from_line = selection.click_line;
			selection.from_item = selection.click_item;
			selection.from_char = selection.click_char;
			selection.to_frame = c_frame;
			selection.to_line = c_line;
			selection.to_item = c_item;
			selection.to_char = c_index;
			bool swap = false;
			if (selection.click_frame && c_frame) {
				const Line &l1 = c_frame->lines[c_line];
				const Line &l2 = selection.click_frame->lines[selection.click_line];
				if (l1.char_offset + c_index < l2.char_offset + selection.click_char) {
					swap = true;
				} else if (l1.char_offset + c_index == l2.char_offset + selection.click_char) {
					deselect();
					return;