Item Disappeared after picking it up

I was exploring some Ancient Cities in my world, as I was putting recently obtained loot into a Shulker Box, and went to mine it. A sound indicated that I picked it up, but I don't see it. As I went to mine gravel, I notice that it went into the next inventory slot, dispite mining gravel with my hand. I went to put something in that specific inventory slot and the item went back into its original spot. I tried it again and again and it wouldn't work. I have a video of it, but since I don't have a computer, and my phone's space is slow.. I had to put it on YouTube.

 

https://youtu.be/7fLts-zt2EA?si=JRMvWbbyETFwK0PE