### 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)

TextServer::StructuredTextParser st_parser_type = TextServer::STRUCTURED_TEXT_DEFAULT;
			BitField<TextServer::JustificationFlag> jst_flags = default_jst_flags;
			for (int i = 0; i < subtag.size(); i++) {
				Vector<String> subtag_a = subtag[i].split("=");
				_normalize_subtags(subtag_a);
				if (subtag_a.size() == 2) {
					if (subtag_a[0] == "justification_flags" || subtag_a[0] == "jst") {
						Vector<String> subtag_b = subtag_a[1].split(",");
						for (const String &E : subtag_b) {
							if (E == "kashida" || E == "k") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_KASHIDA);
							} else if (E == "word" || E == "w") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_WORD_BOUND);
							} else if (E == "trim" || E == "tr") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_TRIM_EDGE_SPACES);
							} else if (E == "after_last_tab" || E == "lt") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_AFTER_LAST_TAB);
							} else if (E == "skip_last" || E == "sl") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_SKIP_LAST_LINE);
							} else if (E == "skip_last_with_chars" || E == "sv") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_SKIP_LAST_LINE_WITH_VISIBLE_CHARS);
							} else if (E == "do_not_skip_singe" || E == "ns") {
								jst_flags.set_flag(TextServer::JUSTIFICATION_DO_NOT_SKIP_SINGLE_LINE);
							}
						}
					} else if (subtag_a[0] == "tab_stops") {
						Vector<String> splitters;
						splitters.push_back(",");
						splitters.push_back(";");
						tab_stops = subtag_a[1].split_floats_mk(splitters);