### 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)

int processed_glyphs = 0;
			while (ofs.y < size.height && from_line < to_line) {
				MutexLock lock(main->lines[from_line].text_buf->get_mutex());
				visible_paragraph_count++;
				visible_line_count += _draw_line(main, from_line, ofs, text_rect.size.x, theme_cache.default_color, theme_cache.outline_size, theme_cache.font_outline_color, theme_cache.font_shadow_color, theme_cache.shadow_outline_size, shadow_ofs, processed_glyphs);
				ofs.y += main->lines[from_line].text_buf->get_size().y + main->lines[from_line].text_buf->get_line_count() * theme_cache.line_separation;
				from_line++;
			}
		} break;
		case NOTIFICATION_INTERNAL_PROCESS: {
			if (is_visible_in_tree()) {
				if (!is_ready()) {
					return;
				}
				double dt = get_process_delta_time();
				_update_fx(main, dt);
				queue_redraw();
			}
		} break;
		case NOTIFICATION_FOCUS_EXIT: {
			if (deselect_on_focus_loss_enabled) {
				deselect();
			}
		} break;
		case NOTIFICATION_DRAG_END: {
			selection.drag_attempt = false;
		} break;
	}
}
Control::CursorShape RichTextLabel::get_cursor_shape(const Point2 &p_pos) const {