### 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 (rtl) {
			prefix = prefix + segment;
		} else {
			prefix = segment + prefix;
		}
	}
	
	int dc_lines = l.text_buf->get_dropcap_lines();
	float h_off = l.text_buf->get_dropcap_size().x;
	if (l.dc_ol_size > 0) {
		l.text_buf->draw_dropcap_outline(ci, p_ofs + ((rtl) ? Vector2() : Vector2(l.offset.x, 0)), l.dc_ol_size, l.dc_ol_color);
	}
	l.text_buf->draw_dropcap(ci, p_ofs + ((rtl) ? Vector2() : Vector2(l.offset.x, 0)), l.dc_color);
	int line_count = 0;
	Size2 ctrl_size = get_size();
	
	for (int line = 0; line < l.text_buf->get_line_count(); line++) {
		if (line > 0) {
			off.y += theme_cache.line_separation;
		}
		if (p_ofs.y + off.y >= ctrl_size.height) {
			break;
		}
		const Size2 line_size = l.text_buf->get_line_size(line);
		if (p_ofs.y + off.y + line_size.y <= 0) {
			off.y += line_size.y;
			continue;
		}
		float width = l.text_buf->get_width();
		float length = line_size.x;