You may have noticed that the Escalation (Level 0) function missing for the user if a group is added as the approver.
1) Set Level 0: Allow to escalate, but assign a group instead of a user object;
The user member of the given group can still approve the task, but is unable to escalate. But if you add the same user directly (not via a group), the escalation task is then available.
This is a product defect (TF00734280).
Individually assigned each desired user objects as Approvers instead of using a group to do so.
This will be fixed in the next public hotfix for 7.1 which is expected to be release around mid/end of December 2017.
Please refer to this article for updates or contact support referencing the product defect: TF00734280.