### 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)

DefaultFont def_font = NORMAL_FONT;
			ItemFont *font_it = _find_font(current);
			if (font_it) {
				if (font_it->font.is_valid()) {
					font = font_it->font;
					def_font = font_it->def_font;
				}
			}
			Ref<FontVariation> fc;
			fc.instantiate();
			int fnt_size = -1;
			for (int i = 1; i < subtag.size(); i++) {
				Vector<String> subtag_a = subtag[i].split("=", true, 1);
				_normalize_subtags(subtag_a);
				if (subtag_a.size() == 2) {
					if (subtag_a[0] == "name" || subtag_a[0] == "n") {
						String fnt = subtag_a[1];
						Ref<Font> font_data = ResourceLoader::load(fnt, "Font");
						if (font_data.is_valid()) {
							font = font_data;
							def_font = CUSTOM_FONT;
						}
					} else if (subtag_a[0] == "size" || subtag_a[0] == "s") {
						fnt_size = subtag_a[1].to_int();
					} else if (subtag_a[0] == "glyph_spacing" || subtag_a[0] == "gl") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_GLYPH, spacing);
					} else if (subtag_a[0] == "space_spacing" || subtag_a[0] == "sp") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_SPACE, spacing);