### 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 (!(p_search_previous && char_idx < 0) &&
				_search_line(selection.from_frame, selection.from_line, p_string, char_idx, p_search_previous)) {
			scroll_to_selection();
			queue_redraw();
			return true;
		}
		char_idx = p_search_previous ? -1 : 0;
		
		if (selection.from_frame->parent != nullptr && selection.from_frame->parent->type == ITEM_TABLE) {
			
			ItemTable *parent_table = static_cast<ItemTable *>(selection.from_frame->parent);
			List<Item *>::Element *parent_element = p_search_previous ? parent_table->subitems.back() : parent_table->subitems.front();
			while (parent_element->get() != selection.from_frame) {
				parent_element = p_search_previous ? parent_element->prev() : parent_element->next();
				ERR_FAIL_NULL_V(parent_element, false);
			}
			
			if (!(p_search_previous && parent_element == parent_table->subitems.front()) &&
					parent_element != parent_table->subitems.back()) {
				parent_element = p_search_previous ? parent_element->prev() : parent_element->next(); 
				ERR_FAIL_NULL_V(parent_element, false);
				
				if (_search_table(parent_table, parent_element, p_string, p_search_previous)) {
					scroll_to_selection();
					queue_redraw();
					return true;
				}
			}
		}
		ending_line = selection.from_frame->line + selection.from_line;