SMS will have assignment_group.name value as empty
This is a known issue in the platform and a PRB was logged PRB1253516 and the issue is fixed in London
The issue occurs because there is another workflow scratchpad variable named "assignment_group" in the If activity of the workflow named "More escalation levels available". The "sys_id" of the assignment group is assigned to that variable.
Due to this reason, the "Send SMS" activity will grab the value of the workflow scratchpad variable (which is the sys_id) instead of the current record's field value.
So to avoid this issue, define another workflow scratchpad variable in the if activity "More escalation levels available" and assign that variable the name of the assignment group and reference that scratchpad variable in the "Send SMS" activity.