### 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)

ItemFrame *frame = static_cast<ItemFrame *>(E);
							int col = idx % col_count;
							int row = idx / col_count;
							if (frame->lines.size() != 0 && row < row_count) {
								Vector2 coff = frame->lines[0].offset;
								if (rtl) {
									coff.x = rect.size.width - table->columns[col].width - coff.x;
								}
								if (row % 2 == 0) {
									Color c = frame->odd_row_bg != Color(0, 0, 0, 0) ? frame->odd_row_bg : odd_row_bg;
									if (c.a > 0.0) {
										draw_rect(Rect2(p_ofs + rect.position + off + coff - frame->padding.position, Size2(table->columns[col].width + h_separation + frame->padding.position.x + frame->padding.size.x, table->rows[row])), c, true);
									}
								} else {
									Color c = frame->even_row_bg != Color(0, 0, 0, 0) ? frame->even_row_bg : even_row_bg;
									if (c.a > 0.0) {
										draw_rect(Rect2(p_ofs + rect.position + off + coff - frame->padding.position, Size2(table->columns[col].width + h_separation + frame->padding.position.x + frame->padding.size.x, table->rows[row])), c, true);
									}
								}
								Color bc = frame->border != Color(0, 0, 0, 0) ? frame->border : border;
								if (bc.a > 0.0) {
									draw_rect(Rect2(p_ofs + rect.position + off + coff - frame->padding.position, Size2(table->columns[col].width + h_separation + frame->padding.position.x + frame->padding.size.x, table->rows[row])), bc, false);
								}
							}
							for (int j = 0; j < (int)frame->lines.size(); j++) {
								_draw_line(frame, j, p_ofs + rect.position + off + Vector2(0, frame->lines[j].offset.y), rect.size.x, p_base_color, p_outline_size, p_outline_color, p_font_shadow_color, p_shadow_outline_size, p_shadow_ofs, r_processed_glyphs);
							}
							idx++;
						}
					} break;