I experienced the same issue. I confirm putting items into box does work but this is just a workaround.
I think this is bug and items should be kept because this happens when I use xbox account in both local computer and BDS.
So I suggest do not differentiate players based on local and network connection if they use the same account.
wait it is /summon falling_block ~~ ~1 ~ {Motion:[0.0,1.0,0.0],BlockState:{Name:sand}} in all the command blocks. activate them excactly at the same time (or just click em all)~
edit:included BlockState tag for better nbt understanding
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I experienced the same issue. I confirm putting items into box does work but this is just a workaround.
I think this is bug and items should be kept because this happens when I use xbox account in both local computer and BDS.
So I suggest do not differentiate players based on local and network connection if they use the same account.