Items disappear when upgrading inventory slots

[Vintarian]

When upgrading inventory slots to the next level, ie reed basket to leather bag, if you have items in your inventory they disappear but if you re-equip the old item they reappear.

[Streetwind]

Not a bug. The items are actually stored inside your baskets/backpacks/sacks etc that you have equipped to provide inventory space.

In real life, if you carry your laptop inside a backpack, and then put the backpack down on the ground somewhere, you no longer have your laptop with you. Because it's in the backpack, on the ground.

Same thing in Vintage Story. What you think of as your inventory is actually just a summary of the contents of the containers you have equipped in the four container slots in the hotbar.

And the position of the slots are not random, either. If you have a linen sack and two reed baskets equipped, then - starting at the top left and going row by row - the first five slots come from the bag, the next three slots will be from the first reed basket, and the last three slots will be from the second reed basket. If you were to reorder the containers on your hotbar, for example putting a reed basket in the first slot, the bag in the second slot, and the second reed basket in the third, then that will also be the order they appear in your inventory view. First three slots for the first basket, then five slots for the bag, then another three for the second basket.

Try and swap the positions of your bags in your hotbar while they have items in them. You'll see that the order they appear in your inventory changes as well.