### 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)

if (!subtag_a[1].is_empty()) {
							Vector<String> feature_tags = subtag_a[1].split(",");
							for (int j = 0; j < feature_tags.size(); j++) {
								Vector<String> subtag_b = feature_tags[j].split("=");
								_normalize_subtags(subtag_b);
								if (subtag_b.size() == 2) {
									features[TS->name_to_tag(subtag_b[0])] = subtag_b[1].to_float();
								} else if (subtag_b.size() == 1) {
									features[TS->name_to_tag(subtag_b[0])] = 1;
								}
							}
							fc->set_opentype_features(features);
						}
					}
				}
			}
			fc->set_base_font(font);
			if (def_font != CUSTOM_FONT) {
				_push_def_font_var(def_font, fc, fnt_size);
			} else {
				push_font(fc, fnt_size);
			}
			pos = brk_end + 1;
			tag_stack.push_front("font");
		} else if (tag.begins_with("outline_size=")) {
			int fnt_size = tag.substr(13, tag.length()).to_int();
			if (fnt_size > 0) {
				push_outline_size(fnt_size);
			}
			pos = brk_end + 1;