Client: 1.1.11 (Steam PC)
Problem: I used Magic Weapon (Paladin, not upcast) on my Ranger's mundane Longbow multiple times while travelling over the map and encountering ambushes or going through various story areas. After doing so at least 2-3 times I realized that the Longbow still had the magic weapon enchantment. It kept the enchantment for the rest of my playthrough. I'm sorry I can't provide better reproduction steps but I did not try to reproduce this again.