Already was reported but with "can't repeat" status and appearance in the game.
As I got know more commonly presents after sea sailing but we have it on the surface also. Just pick up several stacks in the main inventory. After that start to collect them into rucksacks slots. There is no sound of picking but animation still presents. Also, u can't use those invisible items into crafting like they never existed. When u try to place something into the "free" slots collected items appears.
1. Collect some items into character inventory
1.2* Step on the boat
2. Let them go into the rucksack slots