### 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)

Size2 img_size = img->size;
				if (img->size_in_percent) {
					img_size = _get_image_size(img->image, p_width * img->rq_size.width / 100.f, p_width * img->rq_size.height / 100.f, img->region);
				}
				l.text_buf->add_object((uint64_t)it, img_size, img->inline_align, 1);
				txt += String::chr(0xfffc);
				l.char_count++;
				remaining_characters--;
			} break;
			case ITEM_TABLE: {
				ItemTable *table = static_cast<ItemTable *>(it);
				int col_count = table->columns.size();
				int t_char_count = 0;
				
				for (int i = 0; i < col_count; i++) {
					table->columns[i].min_width = 0;
					table->columns[i].max_width = 0;
					table->columns[i].width = 0;
				}
				
				const int available_width = p_width - theme_cache.table_h_separation * (col_count - 1);
				int idx = 0;
				for (Item *E : table->subitems) {
					ERR_CONTINUE(E->type != ITEM_FRAME); 
					ItemFrame *frame = static_cast<ItemFrame *>(E);
					int column = idx % col_count;
					float prev_h = 0;
					for (int i = 0; i < (int)frame->lines.size(); i++) {
						MutexLock sub_lock(frame->lines[i].text_buf->get_mutex());
						int char_offset = l.char_offset + l.char_count;