When trying to use the slider for any pack on a Realm on Nintendo switch the slider goes back to maximum resolution as soon as you leave the settings screen to load up the realm to play on it. In order to recreate this issue all it takes is going into the settings for a texture pack you have set on the realm, not your global texture pack but the world's texture pack. The texture pack needs to have a 128x setting. It will let you slide it to the other resolution options but as soon as you back out of the realms settings it will switch it back to maximum resolution. You don't even need to hit play. But if you do the game will crash since the switch cannot handle 128x textures due to how little RAM it has.