Tested on Fabric 1.20.1 with only dependencies.

Plague of the Longsword should apply Mining Fatigue in place of Haste for those already affected with the "positive" status effect. However, no effect is applied at all.

Other notes: Mjolnir is suboptimal when fighting single targets, as the lightning almost never activates;
Only triggers if target is at least five blocks away from user so as to not damage said user (Github Issue #1 ).

This can almost never trigger, especially if target(s) are chasing the user i.e. closing the five-block gap. Consider granting the user immunity to "lightning_bolt" damage types when holding Mjolnir in the main hand

Your main issue has been fixed in 1.21.1, and probably will not be backported as 1.20.1 is a dead version for development.