Not sure if intended behavior but Magic Weapon is not supposed to work on already magic weapons. Although it occurs that the spell description - in game - doesn't list that as a limitation.
![[Linked Image from i.imgur.com]](https://i.imgur.com/P6OtkUo.png)
Magic Weapon
2 transmutation
Casting Time: 1 bonus action
Range: Touch
Target: A nonmagical weapon
Components: V S
Duration: Concentration, Up to 1 hour
Classes: Paladin, Wizard
You touch a
nonmagical weapon. Until the spell ends, that weapon becomes a magic weapon with a +1 bonus to attack rolls and damage rolls.
At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the bonus increases to +2. When you use a spell slot of 6th level or higher, the bonus increases to +3.