Had some items turn invisible when putting them in backpacks for around 2 updates now.
Happens on land but difficult to reproduce. On the boat it's pretty guaranteed.
It could be related to another bug I'll treat separately.
Have full personal inventory.
Have empty backpack, or backpack with some space availble.
Go on boat with a spare backpack on deck.
Pick up item, the animation will play.
The item might appear in your backpack. It might not.
Pick up the backpack on the deck.
Repeat pick up item step, the animation will play.
The item won't appear in the backpack, but the space is accounted for.
The items you picked up that were invisible are now visible.
If this doesn't replicate the issue, travel out to see and repeat all steps while in motion.