### 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_click.y >= rect.position.y && p_click.y <= rect.position.y + rect.size.y) {
			if ((!rtl && p_click.x >= rect.position.x) || (rtl && p_click.x <= rect.position.x + rect.size.x)) {
				if (p_meta) {
					int64_t glyph_idx = TS->shaped_text_hit_test_grapheme(rid, p_click.x - rect.position.x);
					if (glyph_idx >= 0) {
						const Glyph *glyphs = TS->shaped_text_get_glyphs(rid);
						char_pos = glyphs[glyph_idx].start;
					}
				} else {
					char_pos = TS->shaped_text_hit_test_position(rid, p_click.x - rect.position.x);
					char_pos = TS->shaped_text_closest_character_pos(rid, char_pos);
				}
			}
			line_clicked = true;
			text_rect_begin = rtl ? rect.position.x + rect.size.x : rect.position.x;
		}
		
		if (table_hit && (((char_pos + p_frame->lines[p_line].char_offset) >= table_range.x && (char_pos + p_frame->lines[p_line].char_offset) <= table_range.y) || char_pos == -1)) {
			if (r_click_frame != nullptr) {
				*r_click_frame = table_click_frame;
			}
			if (r_click_line != nullptr) {
				*r_click_line = table_click_line;
			}
			if (r_click_item != nullptr) {
				*r_click_item = table_click_item;
			}
			if (r_click_char != nullptr) {
				*r_click_char = table_click_char;