I think your DM gave you one here.
Several mistakes:
1) Darkness is only 30 foot wide. A foe could get out of it in a single round.
2) Darkness does not halve the speed of foes. So, feared enemies would Dash through it at normal pace (60 feet for most foes). The Wizard should not have been able to beat them around the alley. He could have moved through the alley and stayed up with them, but around seems too much.
3) Fear gives an additional saving throw if the target cannot see the caster. So, the NPCs should have gotten a saving throw every single round any time they are either in the Darkness, or they were on the other side of the Darkness from the wizard. For example, Wizard casts Fear, every NPC that failed the save would get another save at the end of their turn at least for that first round.