### 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)

Item *it = reinterpret_cast<Item *>((uint64_t)objects[i]);
			if (it != nullptr) {
				Rect2 rect = TS->shaped_text_get_object_rect(rid, objects[i]);
				rect.position += p_ofs + off;
				if (p_click.y >= rect.position.y && p_click.y <= rect.position.y + rect.size.y) {
					switch (it->type) {
						case ITEM_TABLE: {
							ItemTable *table = static_cast<ItemTable *>(it);
							int idx = 0;
							int col_count = table->columns.size();
							int row_count = table->rows.size();
							for (Item *E : table->subitems) {
								ItemFrame *frame = static_cast<ItemFrame *>(E);
								int col = idx % col_count;
								int row = idx / col_count;
								if (frame->lines.size() != 0 && row < row_count) {
									Vector2 coff = frame->lines[0].offset;
									if (rtl) {
										coff.x = rect.size.width - table->columns[col].width - coff.x;
									}
									Rect2 crect = Rect2(rect.position + coff - frame->padding.position, Size2(table->columns[col].width + theme_cache.table_h_separation, table->rows[row] + theme_cache.table_v_separation) + frame->padding.position + frame->padding.size);
									if (col == col_count - 1) {
										if (rtl) {
											crect.size.x = crect.position.x + crect.size.x;
											crect.position.x = 0;
										} else {
											crect.size.x = get_size().x;
										}
									}
									if (crect.has_point(p_click)) {