### 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 (width_option->value.ends_with("%")) {
							size_in_percent = true;
						}
					}
					OptionMap::Iterator height_option = bbcode_options.find("height");
					if (height_option) {
						height = height_option->value.to_int();
						if (height_option->value.ends_with("%")) {
							size_in_percent = true;
						}
					}
					OptionMap::Iterator tooltip_option = bbcode_options.find("tooltip");
					if (tooltip_option) {
						tooltip = tooltip_option->value;
					}
					OptionMap::Iterator pad_option = bbcode_options.find("pad");
					if (pad_option) {
						pad = (pad_option->value == "true");
					}
				}
				add_image(texture, width, height, color, (InlineAlignment)alignment, region, Variant(), pad, tooltip, size_in_percent);
			}
			pos = end;
			tag_stack.push_front(bbcode_name);
		} else if (tag.begins_with("color=")) {
			String color_str = tag.substr(6, tag.length()).unquote();
			Color color = Color::from_string(color_str, theme_cache.default_color);
			push_color(color);
			pos = brk_end + 1;
			tag_stack.push_front("color");