Noob question about instancing and variables

Hi all, I'm new to Godot and recently returned to my project that I have started last year. I've been trying to create a reusable button and use it in my GUI scenes. I created a separate scene round_button.tscn which has the following structure:

Control
 └─ RoundTextureButton
     └─ CenterContainer
        └─ IconTextureRect

RoundTextureButton is a TextureButton that reacts to mouse hover and IconTextureRect is a TextureRect that is meant to be an icon with texture set as an AtlasTexture. I made the following script that is attached to Control:

extends Control
class_name RoundButton

enum IconID { play, pause, back, forward, … }

const ICON_REGIONS =  [
	Vector2i(64, 0),
	Vector2i(64, 16),
	…]

@export var icon_id: IconID = IconID.play
@onready var icon_rect: TextureRect = $RoundTextureButton/CenterContainer/IconTextureRect

func _ready() -> void:
	icon_rect.texture.region = Rect2(ICON_REGIONS[icon_id], Vector2i(16, 16))

This creates a dropdown in Godot GUI which allows me to comfortably set the type of a button. It works as expected when I run the button scene.

Then I Instantiate Child Scene… in my menu scene menu.tscn to add a few buttons to my game menu. One button works fine, but when I add two or more, they all have the same icon, the last button that I added and set. What am I missing here with sharing attributes? I read that exported arrays are shared by all instances because the variables are really pointers to some memory location, but my icon_id is an enum value. To test this I have put a print statement in _ready() and it seems like the buttons have distinct values, but the icons are still shared by all of them.