### 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)

_validate_line_caches();
	int line_count = 0;
	int to_line = main->first_invalid_line.load();
	for (int i = 0; i < to_line; i++) {
		MutexLock lock(main->lines[i].text_buf->get_mutex());
		int char_offset = main->lines[i].char_offset;
		int char_count = main->lines[i].char_count;
		if (char_offset <= p_char && p_char < char_offset + char_count) {
			int lc = main->lines[i].text_buf->get_line_count();
			for (int j = 0; j < lc; j++) {
				Vector2i range = main->lines[i].text_buf->get_line_range(j);
				if (char_offset + range.x <= p_char && p_char < char_offset + range.y) {
					break;
				}
				if (char_offset + range.x > p_char && line_count > 0) {
					line_count--; // 
					break;
				}
				if (j != lc - 1) {
					line_count++;
				}
			}
			return line_count;
		} else {
			line_count += main->lines[i].text_buf->get_line_count();
		}
	}
	return -1;
}
int RichTextLabel::get_character_paragraph(int p_char) {