Hitting Escape while renaming a custom label should abort the changes and revert to the default label value.
If you press the cancel button on the bottom, it'll undo any edits you make in the item. I'll ask if we can do undo per field instead of escaping the field.
Note that when creating a new field/section, there are no default labels, as those are default placeholder that are not actual data in the item itself. So, it'd remain blank for now. We'll see if we can improve this.