I was swimming on a one-block wide and one-block high (1x1) water tunnel as it is the path to my storage room. I accidentally dropped one of my shulker boxes and I couldn't get it back. I tried this too with other items and the same happened. So, I started destroying the tunnel and digging around but the items were nowhere to be found.

How to Replicate:

1. Make a one block high and one block wide water tunnel filled with water source blocks.

2. Swim inside the water tunnel.

3. Abruptly stop swimming then drop an item from your inventory.

Observed Results:

1. The item is not returned to your inventory.

2. Dig the blocks surrounding your tunnel but the item is still nowhere to be found.

Expected Results:

1. The item will be floating inside the water tunnel and will return to your inventory.

Video Link:

Disappeared Item While Swimming