ThreadPoolExecutor with wait=True shuts down too early

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-03-27.23:21:42.924>
labels = ['3.8', 'library']
title = 'ThreadPoolExecutor with wait=True shuts down too early'
updated_at = <Date 2020-04-16.08:29:21.191>
user = 'https://github.com/fireattack'

bugs.python.org fields:

activity = <Date 2020-04-16.08:29:21.191>
actor = 'fireattack'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-03-27.23:21:42.924>
creator = 'fireattack'
dependencies = []
files = []
hgrepos = []
issue_num = 40093
keywords = []
message_count = 7.0
messages = ['365194', '365259', '365261', '365269', '365270', '365298', '366582']
nosy_count = 2.0
nosy_names = ['fireattack', 'gaoxinge']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue40093'
versions = ['Python 3.8']