Changes to events are not being saved to XML on save_all()

```py
for vol in anthology.volumes():
if vol.year in ('2018','2019') and "workshop" in str(vol.title).lower() and "student research workshop" not in str(vol.title).lower():
venues = vol.venues()
venue_ids = list(vol.venue_ids)
for v in venues:
if v.type!="workshop":
event = anthology.get_event(f'{v.acronym.lower()}-{vol.year}')
event.add_colocated(vol)
venue_ids.remove(v.id)
vol.venue_ids = tuple(venue_ids)
anthology.save_all()
```

Result: The venue tags are removed but colocated volume info is not added in the XML.