As per the spell description, Amplify lowers attack speeds and spell casting speeds. However, sometimes after the buff is done channeling, my characters would instead gain attack speeds equal to the buff. As a result, they would attack slower by as much as the buff initially gave.
For example, if Bob had an attack speed of 2.2 and got an Amplify attack speed bonus of -1.0, he would have a combined attack speed of 1.2 (which is working as intended). However, I’ve noticed after some fights where I used Amplify, Bob in this case would sometimes have an attack speed of 3.2 without any other changes to his gear. Coincidentally, he was swinging much slower by the same amount the buff initially gave (his slower swings reflected in real time when fighting enemies).
I’ve been able to fix the problem so far by exiting and re-entering a map.