### 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)

for (int j = 0; j < (int)frame->lines.size(); j++) {
											_find_click_in_line(frame, j, rect.position + Vector2(frame->padding.position.x, frame->lines[j].offset.y), rect.size.x, p_click, &table_click_frame, &table_click_line, &table_click_item, &table_click_char, true, p_meta);
											if (table_click_frame && table_click_item) {
												
												table_range = Vector2i(INT32_MAX, 0);
												for (Item *F : table->subitems) {
													ItemFrame *sub_frame = static_cast<ItemFrame *>(F);
													for (int k = 0; k < (int)sub_frame->lines.size(); k++) {
														table_range.x = MIN(table_range.x, sub_frame->lines[k].char_offset);
														table_range.y = MAX(table_range.y, sub_frame->lines[k].char_offset + sub_frame->lines[k].char_count);
													}
												}
												table_offy = off.y;
												table_hit = true;
											}
										}
									}
								}
								idx++;
							}
						} break;
						default:
							break;
					}
				}
			}
		}
		Rect2 rect = Rect2(p_ofs + off - Vector2(0, TS->shaped_text_get_ascent(rid)) - p_frame->padding.position, TS->shaped_text_get_size(rid) + p_frame->padding.position + p_frame->padding.size);
		if (p_table) {
			rect.size.y += theme_cache.table_v_separation;