The first time I killed Atheon on Hard Mode my Primary Weapon inventory was full without me realizing it. Later on when I went to the Postmaster he had a lost primary weapon waiting for me (see image). Correlation does not imply causation. Loot is random. If there is an inventory overflow system in place like this, why would you ever receive loot based on what empty (or full) slots you have? If this has actually happened to you, congratulations on winning the dice roll. Praise RNGesus!