### 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)

table->rows_baseline.clear();
				Vector2 offset;
				float row_height = 0.0;
				for (Item *E : table->subitems) {
					ERR_CONTINUE(E->type != ITEM_FRAME); 
					ItemFrame *frame = static_cast<ItemFrame *>(E);
					int column = idx % col_count;
					offset.x += frame->padding.position.x;
					float yofs = frame->padding.position.y;
					float prev_h = 0;
					float row_baseline = 0.0;
					for (int i = 0; i < (int)frame->lines.size(); i++) {
						MutexLock sub_lock(frame->lines[i].text_buf->get_mutex());
						frame->lines[i].text_buf->set_width(table->columns[column].width);
						table->columns[column].width = MAX(table->columns[column].width, ceil(frame->lines[i].text_buf->get_size().x));
						frame->lines[i].offset.y = prev_h;
						float h = frame->lines[i].text_buf->get_size().y + (frame->lines[i].text_buf->get_line_count() - 1) * theme_cache.line_separation;
						if (i > 0) {
							h += theme_cache.line_separation;
						}
						if (frame->min_size_over.y > 0) {
							h = MAX(h, frame->min_size_over.y);
						}
						if (frame->max_size_over.y > 0) {
							h = MIN(h, frame->max_size_over.y);
						}
						yofs += h;
						prev_h = frame->lines[i].offset.y + frame->lines[i].text_buf->get_size().y + frame->lines[i].text_buf->get_line_count() * theme_cache.line_separation;
						frame->lines[i].offset += offset;
						row_baseline = MAX(row_baseline, frame->lines[i].text_buf->get_line_ascent(frame->lines[i].text_buf->get_line_count() - 1));