I think this is a regression, perhaps caused by the events of UIU-98.
Settings -> Users -> Permission sets
Click on an existing set: its subpermissions are correctly rendered
Click on another existing set: its subpermissions are also correctly rendered
Click on patron-group settings, and return to permission-sets: subpermissions are still correctly rendered
So far so good.
Edit any field of a permission set – it needn't be adding or deleting a subpermission
Click again on the same of the same permission set: now the names of the subpermissions vanish.