### 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)

float faded_visibility = 1.0f;
				if (glyphs[i].start >= fade->starting_index) {
					faded_visibility -= (float)(glyphs[i].start - fade->starting_index) / (float)fade->length;
					faded_visibility = faded_visibility < 0.0f ? 0.0f : faded_visibility;
				}
				font_color.a = faded_visibility;
			}
			bool txt_visible = (font_color.a != 0);
			Transform2D char_xform;
			char_xform.set_origin(p_ofs + off);
			for (int j = 0; j < fx_stack.size(); j++) {
				ItemFX *item_fx = fx_stack[j];
				bool cn = cprev_cluster || (cprev_conn && item_fx->connected);
				if (item_fx->type == ITEM_CUSTOMFX && custom_fx_ok) {
					ItemCustomFX *item_custom = static_cast<ItemCustomFX *>(item_fx);
					Ref<CharFXTransform> charfx = item_custom->char_fx_transform;
					Ref<RichTextEffect> custom_effect = item_custom->custom_effect;
					if (!custom_effect.is_null()) {
						charfx->elapsed_time = item_custom->elapsed_time;
						charfx->range = Vector2i(l.char_offset + glyphs[i].start, l.char_offset + glyphs[i].end);
						charfx->relative_index = l.char_offset + glyphs[i].start - item_fx->char_ofs;
						charfx->visibility = txt_visible;
						charfx->outline = false;
						charfx->font = frid;
						charfx->glyph_index = gl;
						charfx->glyph_flags = gl_fl;
						charfx->glyph_count = gl_cn;
						charfx->offset = fx_offset;
						charfx->color = font_color;
						charfx->transform = char_xform;