### 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)

Item *indentable = p_item;
	while (indentable) {
		if (indentable->type == ITEM_INDENT || indentable->type == ITEM_LIST) {
			return indentable;
		}
		indentable = indentable->parent;
	}
	return indentable;
}
RichTextLabel::ItemFont *RichTextLabel::_find_font(Item *p_item) {
	Item *fontitem = p_item;
	while (fontitem) {
		if (fontitem->type == ITEM_FONT) {
			ItemFont *fi = static_cast<ItemFont *>(fontitem);
			switch (fi->def_font) {
				case NORMAL_FONT: {
					if (fi->variation) {
						Ref<FontVariation> fc = fi->font;
						if (fc.is_valid()) {
							fc->set_base_font(theme_cache.normal_font);
						}
					} else {
						fi->font = theme_cache.normal_font;
					}
					if (fi->def_size) {
						fi->font_size = theme_cache.normal_font_size;
					}
				} break;
				case BOLD_FONT: {
					if (fi->variation) {