When Mining items, it goes away in inventory, even though you mined it!!!

I play on Xbox, but my friend who is on my realm on Nintendo Switch, is getting a really strange, weird bug. 

When he mines, he has an empty slot, and after he mines it, he picks it up, and it is not there! Its a strange bug I do not know why it is happening. If anyone has this bug before on a different platform or on this platform, please message below.