The bug

Item entity merging ignores the Owner tag.

How to reproduce

The following steps require that your UUID is not 00000000-0000-007b-0000-00000000007b (Most: 123L, Least: 123L)

1. Summon an item entity with an Owner

   /summon item ~ ~ ~ {Owner:{M:123L,L:123L},Age:0,Item:{id:"stone",Count:1b}}
   

   → You cannot pick up the item

2. Summon an item entity without Owner

   /summon item ~ ~ ~ {Age:0,Item:{id:"stone",Count:1b}}
   

   → You picked up both items

Code analysis

Based on 1.12.2 decompiled using MCP 9.40

net.minecraft.entity.item.EntityItem.combineItems(EntityItem) ignores the owner.