Alarms get escalated after prior acknowledgement

We have observed that xMatters continue to escalate alarms even after they have been acknowledged. Alerts are sent to the group: Alerts. The screenshot below illustrates the configured Shifts for this group:

The log extracts below illustrates that both User A and User B receive the alarm, despite User A acknowledging the alarm.

06:28:12.565
Responded with Confirm from User A from Android Push Device
06:28:12.719
Response [Confirm] has been mapped to [Confirm] with action [Stop Notifying Target]
06:28:12.817
Terminating notifications for recipients in the TOPS Group.
06:28:12.832
User A - Responded with Confirm

06:39:44.790
Heads - Default Shift was active and will be notified
06:39:44.800
User B has 3 notifiable Device(s)
06:39:44.930
User B - iPhone will be notified
06:39:44.930
User B - Work Phone will be notified
06:39:44.930
User B - Work Email will be notified
06:39:44.930
User B - iPhone is Active
06:39:44.950
User B - iPhone - Notification will be processed by (x)Matters Apple Push PP
06:39:44.950
User B - Work Phone is Active
06:39:44.950
Service Provider (x)matters Voice Gateway does not have a valid and enabled Protocol Provider
06:39:44.950
User B - Work Email is Active
06:39:44.960
User B - Work Email - Notification will be processed by (x)Matters Trial SMTP Email PP

06:40:12.933
Responded with Confirm from User B from Apple Device
06:40:13.068
Response [Confirm] has been mapped to [Confirm] with action [Stop Notifying Target]
06:40:13.152
Terminating notifications for recipients in the Alerts Group.
06:40:13.164
User B - Responded with Confirm