Shulker boxes disappearing from inventory occasionally

So in the past week I have been playing on my “forever world” and have made a lot of accomplishments and done cool things but lately I have been using shulker boxes to store things and I find that they frequently are disappearing from my inventory or becoming invisible as an icon on my hotbar and when invisible if I accidentaly replace them with another block they are gone forever. I have just lost my best one yet full enchanted and maxed out netherite Armour and elytra that took me hours to get and I don't know how I get them back I'd appreciate help because these were really important to my world.