When a hopper sends a kind of items in a chest and the player adds the same kind of items at the exact same time as the hopper, the items initially in the chest are deleted.

Note
The bug was experienced on a realm in survival but works in solo and every game modes.

The bug is a bit hard to reproduce you have to get the timing or brute force it. (Took me 5mins when I recorded it, by brute forcing it)

How to Reproduce

1. In creative mode, take a hopper and 2 chests. One chest input one kind of items in the second one.
2. Lock the hopper to prevent items from being transferred for the moment.
3. Choose an item and fill entirely the top chest (you could try several times and compare the number of items before and after the manipulation).
4. Take the same item in your hands, do little packs of it to have multiple attempts. (the number of items your adding to the one the slot being filled in the chest is superior to 64 the bug seems to not occurs as the stack and the extra items just switches places).
5. It's a matter of timing (or luck), try to reproduce the rhythm of the hopper in your head or out loud. When you think you have it add the items in hand over the ones in chest. Try again until you succeed.
6. Finally, lock the hopper again and take all the items to the top chest to confirm the loss (if you haven’t seen the bug occurred.

Here is a [link|https://clipchamp.com/watch/Lfi9rpnaZ3z] to a video of me reproducing the bug. I succeed the bug at 0:44.
(ClipChamp is a Microsoft Application)

Expected result
Initial:
X: items in the chest
Y: items in your hand
Final:
X+Y+1 in the chest
(+1 is the item send by the hopper)

Observed result
Final:
Y+1 in the chest
X disappeared

As I experienced in solo after your answer. I managed to reproduce the bug several times. And so I tested with 8 items in hand and got 9. Which led me to believe that it was the items in the chest that were disappearing (X). But when experienced the first time by accident I lost 47 scraps so may I lost X+Y? I don’t know. Moreover, to complete my report I decided to add a video and as I tried to reproduce the bug I’ve done it with X+Y > 64, that I thought impossible before. And it resulted to be another result:

40 + 32 + 1 => 24 + 8 (in hand)
And: 24 + 8 = 32

Here the result is Y:
When items are added to another items if the amount is greater than 64, You’ll get the rest in your hand. Here 40 + 32 is 64 + 8. So X disappeared as expected and the items even divided correctly. I think the item send by the hopper disappear too in that case because it’s than greater than 64. Maybe that's what happened when I was playing and experienced the bug.