### 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)

Vector<String> region_values = region_option->value.split(",", false);
					if (region_values.size() == 4) {
						region.position.x = region_values[0].to_float();
						region.position.y = region_values[1].to_float();
						region.size.x = region_values[2].to_float();
						region.size.y = region_values[3].to_float();
					}
				}
				Color color = Color(1.0, 1.0, 1.0);
				OptionMap::Iterator color_option = bbcode_options.find("color");
				if (color_option) {
					color = Color::from_string(color_option->value, color);
				}
				int width = 0;
				int height = 0;
				bool pad = false;
				String tooltip;
				bool size_in_percent = false;
				if (!bbcode_value.is_empty()) {
					int sep = bbcode_value.find("x");
					if (sep == -1) {
						width = bbcode_value.to_int();
					} else {
						width = bbcode_value.substr(0, sep).to_int();
						height = bbcode_value.substr(sep + 1).to_int();
					}
				} else {
					OptionMap::Iterator width_option = bbcode_options.find("width");
					if (width_option) {
						width = width_option->value.to_int();