What components did they replace with the original work? Did they just replace the bumper cover?
If you were charged for a new tail lamp/light assembly in the previous work, then this is on them. If they only pulled the original tail lamp as part of other work, and reinstalled the original tail lamp, then the tail lamp was probably cracked in the collision. That damage would be easy to miss and can take a long time to become apparant. It sounds to me like this component was not replaced in the original work and you are only now discovering damage that was sustained in the collision.
What components did they replace with the original work? Did they just replace the bumper cover?
If you were charged for a new tail lamp/light assembly in the previous work, then this is on them. If they only pulled the original tail lamp as part of other work, and reinstalled the original tail lamp, then the tail lamp was probably cracked in the collision. That damage would be easy to miss and can take a long time to become apparant. It sounds to me like this component was not replaced in the original work and you are only now discovering damage that was sustained in the collision.