Observed: the Impaler +3 spear (dropped by King Ixilthetocal in the City-of-Caverns) shows in the Inventory and Record sheets as doing 1d6+3 damage, as if it were as an ordinary +3 spear. Nevertheless, when used, its extra +10 piercing damage does correctly occur, shown in the dialogue window as a second line of damage-report for each hit.
Expected: according to its description, Impaler +3 should simply do "1d6+13 (piercing)", rather than "1d6+ 3 (piercing), +10 piercing damage". The +10 extra damage should therefore be included in the calculation shown on the Inventory and Record sheets as "Impaler +3: 1d6+13", not excluded from view as a bonus.
TLDR: it does actually work correctly, it's just not displayed correctly in the calculation.