### 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)

switch (p_option) {
		case MENU_COPY: {
			selection_copy();
		} break;
		case MENU_SELECT_ALL: {
			select_all();
		} break;
	}
}
void RichTextLabel::_draw_fbg_boxes(RID p_ci, RID p_rid, Vector2 line_off, Item *it_from, Item *it_to, int start, int end, int fbg_flag) {
	Vector2i fbg_index = Vector2i(end, start);
	Color last_color = Color(0, 0, 0, 0);
	bool draw_box = false;
	// 
	for (int i = start; i < end; i++) {
		Item *it = _get_item_at_pos(it_from, it_to, i);
		Color color;
		if (fbg_flag == 0) {
			color = _find_bgcolor(it);
		} else {
			color = _find_fgcolor(it);
		}
		bool change_to_color = ((color.a > 0) && ((last_color.a - 0.0) < 0.01));
		bool change_from_color = (((color.a - 0.0) < 0.01) && (last_color.a > 0.0));
		bool change_color = (((color.a > 0) == (last_color.a > 0)) && (color != last_color));
		if (change_to_color) {
			fbg_index.x = MIN(i, fbg_index.x);
			fbg_index.y = MAX(i, fbg_index.y);
		}
		if (change_from_color || change_color) {