The Full Auto Shotgun, ACS12 in the loadout of Maestro and Alibi does not count towards Tier Challenges for Shotgun kills. I took 3 kills with her shotgun and still it did not register them. All of the shotguns except the ACS12 does count towards Shotgun kills. This has been an old problem since Steel Wave but there is still no fix for it
We are aware of this issue and it is actively being investigated. Apologies for any inconvenience caused by this.