CharFXTransform - relative_index is substracted by 1 each RichTextLabel bbcode effect block

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Godot version

v4.2.rc1.official [ad72de5]

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 at the RichTextLabel I found a bug regarding the CharFXTransform.relative_index.
The variable is explained as follows in the documentation:

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.

Steps to reproduce

Add this script as Custom Effect to the RichTextLabel (or see the MRP):

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