[BUG] ReferenceError: weakly-referenced object no longer exists

Here's a perplexing mongoengine-related error:

import fiftyone as fo
import fiftyone.zoo as foz

dataset = foz.load_zoo_dataset("quickstart")

# this fails
dataset.first()["tags"].append("tag")

# but this works
sample = dataset.first()
sample["tags"].append("tag")

ReferenceError                            Traceback (most recent call last)
<ipython-input-2-846545a97f03> in <module>
----> 1 dataset.first()['tags'].append('tag')

~/dev/env/fo/lib/python3.9/site-packages/mongoengine/base/datastructures.py in wrapper(self, *args, **kwargs)
     21         # Can't use super() in the decorator.
     22         result = parent_method(self, *args, **kwargs)
---> 23         self._mark_as_changed()
     24         return result
     25 

~/dev/env/fo/lib/python3.9/site-packages/mongoengine/base/datastructures.py in _mark_as_changed(self, key)
    179 
    180     def _mark_as_changed(self, key=None):
--> 181         if hasattr(self._instance, "_mark_as_changed"):
    182             if key is not None:
    183                 self._instance._mark_as_changed(f"{self._name}.{key % len(self)}")

ReferenceError: weakly-referenced object no longer exists