### 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)

_stop_thread();
	MutexLock data_lock(data_mutex);
	int pos = 0;
	List<String> tag_stack;
	int indent_level = 0;
	bool in_bold = false;
	bool in_italics = false;
	bool after_list_open_tag = false;
	bool after_list_close_tag = false;
	set_process_internal(false);
	while (pos <= p_bbcode.length()) {
		int brk_pos = p_bbcode.find("[", pos);
		if (brk_pos < 0) {
			brk_pos = p_bbcode.length();
		}
		String txt = brk_pos > pos ? p_bbcode.substr(pos, brk_pos - pos) : "";
		
		if (after_list_close_tag || after_list_open_tag) {
			txt = txt.trim_prefix("\n");
		}
		if (brk_pos == p_bbcode.length()) {
			
			if (txt.is_empty() && after_list_open_tag) {
				txt = "\n";
			}
			if (!txt.is_empty()) {
				add_text(txt);