### 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)

Vector<ItemFX *> fx_stack;
			_fetch_item_fx_stack(it, fx_stack);
			bool custom_fx_ok = true;
			Point2 fx_offset = Vector2(glyphs[i].x_off, glyphs[i].y_off);
			RID frid = glyphs[i].font_rid;
			uint32_t gl = glyphs[i].index;
			uint16_t gl_fl = glyphs[i].flags;
			uint8_t gl_cn = glyphs[i].count;
			bool cprev_cluster = false;
			bool cprev_conn = false;
			if (gl_cn == 0) { 
				cprev_cluster = true;
			}
			if (gl_fl & TextServer::GRAPHEME_IS_RTL) { 
				if (i > 0 && (glyphs[i - 1].flags & TextServer::GRAPHEME_IS_CONNECTED)) {
					cprev_conn = true;
				}
			} else {
				if (glyphs[i].flags & TextServer::GRAPHEME_IS_CONNECTED) {
					cprev_conn = true;
				}
			}
			
			if (fade) {
				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_outline_color.a = faded_visibility;