This was discovered in ibis-project/ibis#8687

Due to a strict isinstance(x, dict) check, custom mapping types would instead get treated as mere iterables, so only the keys would get printed as a tuple:

import pandas as pd
from ibis.common.collections import frozendict
pd.Series([frozendict({"a": 5, "b": 6})])
# 0    (a, b)
# dtype: object

• NA: closes #xxxx
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• NA Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.