### 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)

tag_stack.push_front("hint");
		} else if (tag.begins_with("dropcap")) {
			Vector<String> subtag = tag.substr(5, tag.length()).split(" ");
			_normalize_subtags(subtag);
			int fs = theme_cache.normal_font_size * 3;
			Ref<Font> f = theme_cache.normal_font;
			Color color = theme_cache.default_color;
			Color outline_color = theme_cache.font_outline_color;
			int outline_size = theme_cache.outline_size;
			Rect2 dropcap_margins;
			for (int i = 0; i < subtag.size(); i++) {
				Vector<String> subtag_a = subtag[i].split("=");
				_normalize_subtags(subtag_a);
				if (subtag_a.size() == 2) {
					if (subtag_a[0] == "font" || subtag_a[0] == "f") {
						String fnt = subtag_a[1];
						Ref<Font> font = ResourceLoader::load(fnt, "Font");
						if (font.is_valid()) {
							f = font;
						}
					} else if (subtag_a[0] == "font_size") {
						fs = subtag_a[1].to_int();
					} else if (subtag_a[0] == "margins") {
						Vector<String> subtag_b = subtag_a[1].split(",");
						_normalize_subtags(subtag_b);
						if (subtag_b.size() == 4) {
							dropcap_margins.position.x = subtag_b[0].to_float();
							dropcap_margins.position.y = subtag_b[1].to_float();
							dropcap_margins.size.x = subtag_b[2].to_float();
							dropcap_margins.size.y = subtag_b[3].to_float();