### 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)

alignment = INLINE_ALIGNMENT_CENTER;
				} else if (subtag[1] == "bottom" || subtag[1] == "b") {
					alignment = INLINE_ALIGNMENT_BOTTOM;
				}
			}
			int row = -1;
			if (subtag.size() > 3) {
				row = subtag[3].to_int();
			}
			push_table(columns, (InlineAlignment)alignment, row);
			pos = brk_end + 1;
			tag_stack.push_front("table");
		} else if (tag == "cell") {
			push_cell();
			pos = brk_end + 1;
			tag_stack.push_front(tag);
		} else if (tag.begins_with("cell=")) {
			int ratio = tag.substr(5, tag.length()).to_int();
			if (ratio < 1) {
				ratio = 1;
			}
			set_table_column_expand(get_current_table_column(), true, ratio);
			push_cell();
			pos = brk_end + 1;
			tag_stack.push_front("cell");
		} else if (tag.begins_with("cell ")) {
			Vector<String> subtag = tag.substr(5, tag.length()).split(" ");
			_normalize_subtags(subtag);
			for (int i = 0; i < subtag.size(); i++) {
				Vector<String> subtag_a = subtag[i].split("=");