Right, so. Immersive Engineering has conveyor belts. More pertinently, Immersive Engineering has covered conveyor belts that prevent you from picking up items that are currently on them while you are standing next to or on the belt.

A while back, there was an issue where during the brief moment immediately a machine inserted an item into a covered conveyor belt, the item could be picked up if you were adjacent to said belt. That problem was eventually addressed here.

Now as it happens, it turns out that Realistic Item Drops, when the config is set to "Auto Pickup"=true, breaks things again, such that if you were to stand next to a belt as you toss an item into the hopper, it ends up right back into your inventory.

Pictured: essentially the same configuration as mentioned in the above linked issue.

Since this problem doesn't occur without RealisticItemDrops present, I figured I should bring the problem to you first.

Tested with immersive engineering 0.12-84, realistic item drops 1.2.12, and forge 2705. No other mods present.