mailbox fails to read message from mbox

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-11-23.18:18:02.343>
labels = ['3.8', 'type-bug', 'library']
title = 'mailbox fails to read message from mbox'
updated_at = <Date 2021-11-23.18:18:02.343>
user = 'https://bugs.python.org/peterwurmsdobler'

bugs.python.org fields:

activity = <Date 2021-11-23.18:18:02.343>
actor = 'peter.wurmsdobler'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-11-23.18:18:02.343>
creator = 'peter.wurmsdobler'
dependencies = []
files = []
hgrepos = []
issue_num = 45882
keywords = []
message_count = 1.0
messages = ['406873']
nosy_count = 1.0
nosy_names = ['peter.wurmsdobler']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45882'
versions = ['Python 3.8']