### 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)

} else if (tag.begins_with("outline_color=")) {
			String color_str = tag.substr(14, tag.length()).unquote();
			Color color = Color::from_string(color_str, theme_cache.default_color);
			push_outline_color(color);
			pos = brk_end + 1;
			tag_stack.push_front("outline_color");
		} else if (tag.begins_with("font_size=")) {
			int fnt_size = tag.substr(10, tag.length()).to_int();
			push_font_size(fnt_size);
			pos = brk_end + 1;
			tag_stack.push_front("font_size");
		} else if (tag.begins_with("opentype_features=") || tag.begins_with("otf=")) {
			int value_pos = tag.find("=");
			String fnt_ftr = tag.substr(value_pos + 1);
			Vector<String> subtag = fnt_ftr.split(",");
			_normalize_subtags(subtag);
			if (subtag.size() > 0) {
				Ref<Font> font = theme_cache.normal_font;
				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;
					}
				}
				Dictionary features;
				for (int i = 0; i < subtag.size(); i++) {
					Vector<String> subtag_a = subtag[i].split("=");
					_normalize_subtags(subtag_a);