Issue
This issue only occurs when "Lock belt dragging to a straight line" and "Smart belt dragging" under Settings>Interface>Interaction are turned on. If either setting is disabled, the issue will not occur. Items on belts disappear when a player drags over them in the opposite direction in between two underground belts. The belts disappearing makes sense, even if unwanted. The belts reappear in the player's inventory. The items are deleted permanently. See attached video (apologies for the music, i didn't realize it captured that)Reproduction
Ensure that "Lock belt dragging to a straight line" and "Smart belt dragging" under Settings>Interface>Interaction are turned on.Place a length of underground belt. Place normal belts in between the underground belts. Place items on the belts. Holding a normal belt facing the opposite direction of the other belts, click and hold in line with the underground belts. Drag across the belts. The belts will disappear into your inventory, but the items on the belts will disappear.
