field "mro" behaves strangely in dataclass

GitHub fields:

assignee = 'https://github.com/ericvsmith'
closed_at = None
created_at = <Date 2021-10-19.23:25:44.087>
labels = ['type-bug', 'library', '3.10']
title = 'field "mro" behaves strangely in dataclass'
updated_at = <Date 2021-10-23.07:20:55.041>
user = 'https://github.com/finite-state-machine'

bugs.python.org fields:

activity = <Date 2021-10-23.07:20:55.041>
actor = 'serhiy.storchaka'
assignee = 'eric.smith'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-10-19.23:25:44.087>
creator = 'finite-state-machine'
dependencies = []
files = []
hgrepos = []
issue_num = 45531
keywords = []
message_count = 9.0
messages = ['404378', '404381', '404414', '404430', '404458', '404819', '404824', '404846', '404857']
nosy_count = 4.0
nosy_names = ['eric.smith', 'eric.araujo', 'serhiy.storchaka', 'finite-state-machine']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45531'
versions = ['Python 3.10']