io.BytesIO.truncate does not work as advertised

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-06-07.20:52:57.361>
labels = ['type-bug', 'library', 'expert-IO']
title = 'io.BytesIO.truncate does not work as advertised'
updated_at = <Date 2018-10-12.10:55:18.233>
user = 'https://github.com/teythoon'

bugs.python.org fields:

activity = <Date 2018-10-12.10:55:18.233>
actor = 'anton.barkovsky'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'IO']
creation = <Date 2016-06-07.20:52:57.361>
creator = 'justus.winter'
dependencies = []
files = []
hgrepos = []
issue_num = 27261
keywords = []
message_count = 5.0
messages = ['267748', '267758', '267765', '267848', '327578']
nosy_count = 7.0
nosy_names = ['alexandre.vassalotti', 'martin.panter', 'serhiy.storchaka', 'anton.barkovsky', 'xiang.zhang', 'justus.winter', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27261'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']