Python 3 f.tell() gets out of sync with file pointer in binary append+read mode

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-03-24.00:33:44.992>
labels = ['3.8', 'type-bug', '3.7', 'expert-IO']
title = 'Python 3 f.tell() gets out of sync with file pointer in binary append+read mode'
updated_at = <Date 2019-06-01.11:04:40.585>
user = 'https://github.com/pewscorner'

bugs.python.org fields:

activity = <Date 2019-06-01.11:04:40.585>
actor = 'cheryl.sabella'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['IO']
creation = <Date 2019-03-24.00:33:44.992>
creator = 'pewscorner'
dependencies = []
files = ['48368', '48370']
hgrepos = []
issue_num = 36411
keywords = ['patch']
message_count = 13.0
messages = ['338709', '338775', '339171', '343340', '343341', '343345', '343742', '343744', '343955', '343969', '344017', '344049', '344151']
nosy_count = 7.0
nosy_names = ['terry.reedy', 'benjamin.peterson', 'martin.panter', 'serhiy.storchaka', 'Windson Yang', 'pewscorner', 'Jeffrey.Kintscher']
pr_nums = ['13717']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36411'
versions = ['Python 3.7', 'Python 3.8']