I filled a dyed pink, named shulker box with buckets of axolotl and buckets of tropical fish, and mined it to pick it up. It appeared in my inventory. I then triggered and completed a raid, as well as killed an enemy which caused skulk to expand (mentioning it here in case these facts are somehow tied to the bug).  When I went back to my inventory,  my shulker box and its contents had completely disappeared. 

I did not die, log out, or  have my game crash during this time. I am playing on a single-player world. I thoroughly searched my entire (small) base area and every chest multiple times. I tried logging out and back in, as well as closing and re-launching MC. Nothing ever re-appeared. 

I saw similar reports, but they were all after a death or crash. this seemed to happen out of the clear blue. It was very disappointing to lose the rare mobs I had in the shulker.