### 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)

}
			push_rainbow(saturation, value, frequency);
			pos = brk_end + 1;
			tag_stack.push_front("rainbow");
			set_process_internal(true);
		} else if (bbcode_name == "pulse") {
			Color color = Color(1, 1, 1, 0.25);
			OptionMap::Iterator color_option = bbcode_options.find("color");
			if (color_option) {
				color = Color::from_string(color_option->value, color);
			}
			float frequency = 1.0;
			OptionMap::Iterator freq_option = bbcode_options.find("freq");
			if (freq_option) {
				frequency = freq_option->value.to_float();
			}
			float ease = -2.0;
			OptionMap::Iterator ease_option = bbcode_options.find("ease");
			if (ease_option) {
				ease = ease_option->value.to_float();
			}
			push_pulse(color, frequency, ease);
			pos = brk_end + 1;
			tag_stack.push_front("pulse");
			set_process_internal(true);
		} else if (tag.begins_with("bgcolor=")) {
			String color_str = tag.substr(8, tag.length()).unquote();
			Color color = Color::from_string(color_str, theme_cache.default_color);
			push_bgcolor(color);
			pos = brk_end + 1;