contextvars.Context.run hangs forever in ProccessPoolExecutor

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-01-08.10:34:34.026>
labels = ['type-bug', '3.8']
title = 'contextvars.Context.run hangs forever in ProccessPoolExecutor'
updated_at = <Date 2020-01-10.19:23:50.063>
user = 'https://github.com/Ronserruya'

bugs.python.org fields:

activity = <Date 2020-01-10.19:23:50.063>
actor = 'yselivanov'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2020-01-08.10:34:34.026>
creator = 'ronserruya'
dependencies = []
files = []
hgrepos = []
issue_num = 39257
keywords = []
message_count = 2.0
messages = ['359575', '359749']
nosy_count = 2.0
nosy_names = ['yselivanov', 'ronserruya']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39257'
versions = ['Python 3.8']