### 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_mask & UPDATE_TEXTURE) {
					if (item->region.has_area()) {
						Ref<AtlasTexture> atlas_tex = memnew(AtlasTexture);
						atlas_tex->set_atlas(p_image);
						atlas_tex->set_region(item->region);
						item->image = atlas_tex;
					} else {
						item->image = p_image;
					}
				}
				if (p_mask & UPDATE_COLOR) {
					item->color = p_color;
				}
				if (p_mask & UPDATE_TOOLTIP) {
					item->tooltip = p_tooltip;
				}
				if (p_mask & UPDATE_PAD) {
					item->pad = p_pad;
				}
				if (p_mask & UPDATE_ALIGNMENT) {
					if (item->inline_align != p_alignment) {
						reshape = true;
						item->inline_align = p_alignment;
					}
				}
				if (p_mask & UPDATE_WIDTH_IN_PERCENT) {
					if (item->size_in_percent != p_size_in_percent) {
						reshape = true;
						item->size_in_percent = p_size_in_percent;