Workflow activity is missing or not displaying some fields.
Fields like Advanced checkbox and Additional approvers scripts are disappearing when you load the "Approval - User" workflow activity in the workflow editor.
This can be caused by missing workflow activity variable [wf_activity_variable] in the Activity Definition
1. Go to Workflow > Administration > Activity Definition
2. Search for the name of your activity and check the Activity Variables tab
3. The variables should be present there to show in the workflow activity
4. If not present, import the variables from an out-of-box (OOB) instance if it is an OOB one
Check deleted records or the Customer Updates table if the variable missing could have been deleted or updated