### 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)

} else if (subtag_a[0] == "top_spacing" || subtag_a[0] == "top") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_TOP, spacing);
					} else if (subtag_a[0] == "bottom_spacing" || subtag_a[0] == "bt") {
						int spacing = subtag_a[1].to_int();
						fc->set_spacing(TextServer::SPACING_BOTTOM, spacing);
					} else if (subtag_a[0] == "embolden" || subtag_a[0] == "emb") {
						float emb = subtag_a[1].to_float();
						fc->set_variation_embolden(emb);
					} else if (subtag_a[0] == "face_index" || subtag_a[0] == "fi") {
						int fi = subtag_a[1].to_int();
						fc->set_variation_face_index(fi);
					} else if (subtag_a[0] == "slant" || subtag_a[0] == "sln") {
						float slant = subtag_a[1].to_float();
						fc->set_variation_transform(Transform2D(1.0, slant, 0.0, 1.0, 0.0, 0.0));
					} else if (subtag_a[0] == "opentype_variation" || subtag_a[0] == "otv") {
						Dictionary variations;
						if (!subtag_a[1].is_empty()) {
							Vector<String> variation_tags = subtag_a[1].split(",");
							for (int j = 0; j < variation_tags.size(); j++) {
								Vector<String> subtag_b = variation_tags[j].split("=");
								_normalize_subtags(subtag_b);
								if (subtag_b.size() == 2) {
									variations[TS->name_to_tag(subtag_b[0])] = subtag_b[1].to_float();
								}
							}
							fc->set_variation_opentype(variations);
						}
					} else if (subtag_a[0] == "opentype_features" || subtag_a[0] == "otf") {
						Dictionary features;