### 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("bgcolor");
		} else if (tag.begins_with("fgcolor=")) {
			String color_str = tag.substr(8, tag.length()).unquote();
			Color color = Color::from_string(color_str, theme_cache.default_color);
			push_fgcolor(color);
			pos = brk_end + 1;
			tag_stack.push_front("fgcolor");
		} else {
			Vector<String> &expr = split_tag_block;
			if (expr.size() < 1) {
				add_text("[");
				pos = brk_pos + 1;
			} else {
				String identifier = expr[0];
				expr.remove_at(0);
				Dictionary properties = parse_expressions_for_values(expr);
				Ref<RichTextEffect> effect = _get_custom_effect_by_code(identifier);
				if (!effect.is_null()) {
					push_customfx(effect, properties);
					pos = brk_end + 1;
					tag_stack.push_front(identifier);
				} else {
					add_text("["); 
					pos = brk_pos + 1;
				}
			}
		}
	}
	Vector<ItemFX *> fx_items;
	for (Item *E : main->subitems) {