The api.write() method replaces any existing codecs.

The collection TypeRegistry is replaced with a new instance, effectively removing any existing custom codecs.
https://github.com/mongodb-labs/mongo-arrow/blob/1.6.0/bindings/python/pymongoarrow/api.py#L419

A related issue is that pymongoarrow uses pyarrow.to_pylist() to convert a pyarrow.Table to Python objects as an intermediate step before converting to raw BSON.

Pyarrow converts Arrow decimal128 types to Python Decimal objects. However, BSON cannot handle Decimal types, necessitating the use of a custom codec.
https://github.com/mongodb-labs/mongo-arrow/blob/1.6.0/bindings/python/pymongoarrow/api.py#L385