### 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)

frid = charfx->font;
						gl = charfx->glyph_index;
						txt_visible &= charfx->visibility;
					}
				} else if (item_fx->type == ITEM_SHAKE) {
					ItemShake *item_shake = static_cast<ItemShake *>(item_fx);
					if (!cn) {
						uint64_t char_current_rand = item_shake->offset_random(glyphs[i].start);
						uint64_t char_previous_rand = item_shake->offset_previous_random(glyphs[i].start);
						uint64_t max_rand = 2147483647;
						double current_offset = Math::remap(char_current_rand % max_rand, 0, max_rand, 0.0f, 2.f * (float)Math_PI);
						double previous_offset = Math::remap(char_previous_rand % max_rand, 0, max_rand, 0.0f, 2.f * (float)Math_PI);
						double n_time = (double)(item_shake->elapsed_time / (0.5f / item_shake->rate));
						n_time = (n_time > 1.0) ? 1.0 : n_time;
						item_shake->prev_off = Point2(Math::lerp(Math::sin(previous_offset), Math::sin(current_offset), n_time), Math::lerp(Math::cos(previous_offset), Math::cos(current_offset), n_time)) * (float)item_shake->strength / 10.0f;
					}
					fx_offset += item_shake->prev_off;
				} else if (item_fx->type == ITEM_WAVE) {
					ItemWave *item_wave = static_cast<ItemWave *>(item_fx);
					if (!cn) {
						double value = Math::sin(item_wave->frequency * item_wave->elapsed_time + ((p_ofs.x + gloff.x) / 50)) * (item_wave->amplitude / 10.0f);
						item_wave->prev_off = Point2(0, 1) * value;
					}
					fx_offset += item_wave->prev_off;
				} else if (item_fx->type == ITEM_TORNADO) {
					ItemTornado *item_tornado = static_cast<ItemTornado *>(item_fx);
					if (!cn) {
						double torn_x = Math::sin(item_tornado->frequency * item_tornado->elapsed_time + ((p_ofs.x + gloff.x) / 50)) * (item_tornado->radius);
						double torn_y = Math::cos(item_tornado->frequency * item_tornado->elapsed_time + ((p_ofs.x + gloff.x) / 50)) * (item_tornado->radius);
						item_tornado->prev_off = Point2(torn_x, torn_y);