If you select an item in the inventory (to move it or drop it), but go through a portal while still having the item selected, the item you selected disappears.

What I expected to happen was...:
This item would show up in my inventory in a place that I took it from after going through the portal, or it would be automatically dropped.

What actually happened was...:
The item disappears.

Steps to Reproduce:
1st method:

1. Build a nether portal and activate it.
2. Build a railway using powered rails (to power the minecart) facing the nether portal and power it with some source of redstone signal (like in the screenshot)
3. Place some items in your inventory
4. Place a minecart at the beginning of a railway.
5. Sit in the minecart and press "Move forward" button while looking at the nether portal to power it.
6. Before going through nether portal, click any item in your inventory to select it and don't put it anywhere.

or

2nd method:

1. Build a end/nether portal and activate it.
2. Build a water stream above/next to the portal so that you go into it at the end of the stream.
3. While being in the water stream, click any item in your inventory to select it and don't put it anywhere.
4. Wait until you go through the portal.

To be sure that the item was not dropped anywhere, you can kill all other items by running /kill @e[type=item] and then run /data get entity @n[type=item] after reproducing the issue.