Fix substitutions import

aleksis/apps/untis/util/mysql/importers/absences.py

@@ -156,7 +156,11 @@ def import_absences(
                     amends=ref_object,
                     datetime_start=affected_event["DTSTART"].dt,
                     datetime_end=affected_event["DTEND"].dt,
-                    defaults=dict(cancelled=True),
+                    defaults=dict(
+                        cancelled=True,
+                        slot_number_start=ref_object.slot_number_start,
+                        slot_number_end=ref_object.slot_number_end,
+                    ),
                 )
                 amending_event.extended_data["cancelled_by_absence_untis"] = import_ref
                 amending_event.save()