### 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 (!prefix.is_empty() && line == 0) {
			Ref<Font> font = theme_cache.normal_font;
			int font_size = theme_cache.normal_font_size;
			ItemFont *font_it = _find_font(l.from);
			if (font_it) {
				if (font_it->font.is_valid()) {
					font = font_it->font;
				}
				if (font_it->font_size > 0) {
					font_size = font_it->font_size;
				}
			}
			ItemFontSize *font_size_it = _find_font_size(l.from);
			if (font_size_it && font_size_it->font_size > 0) {
				font_size = font_size_it->font_size;
			}
			if (rtl) {
				font->draw_string(ci, p_ofs + Vector2(off.x + length, l.text_buf->get_line_ascent(0)), " " + prefix, HORIZONTAL_ALIGNMENT_LEFT, l.offset.x, font_size, _find_color(l.from, p_base_color));
			} else {
				font->draw_string(ci, p_ofs + Vector2(off.x - l.offset.x, l.text_buf->get_line_ascent(0)), prefix + " ", HORIZONTAL_ALIGNMENT_RIGHT, l.offset.x, font_size, _find_color(l.from, p_base_color));
			}
		}
		if (line <= dc_lines) {
			if (rtl) {
				off.x -= h_off;
			} else {
				off.x += h_off;
			}
		}
		RID rid = l.text_buf->get_line_rid(line);