The problem is similar to https://bugs.python.org/issue22931 with square brackets in cookie values.
Incorrect value is serialized JSON in this case, for example:

>>> from Cookie import SimpleCookie
>>> cookie = SimpleCookie()
>>> cookie.load('a=b; c={"somekey":"value"}; d=e, f=g, some=other')
>>> cookie.output()
'Set-Cookie: a=b'
>>> 

From my tests I figured out that quote mark (") is causing problems.

In fact, according to HTTP specification, cookies can't be JSON objects, but I think that python library shouldn't silently allow incorrect values. Probably incorrect cookies should be skipped, or some exception should be thrown there.

Problem exists in Chrome for sure (it sends cookies as they are - set with JS for example).

I have two another examples that can make this issue more clear:

>>> from Cookie import SimpleCookie
>>> c = SimpleCookie()
>>> c.load('a=b; c=c"c; d=d')
>>> c.output()
'Set-Cookie: a=b'

Also using escaping and quotes (cookie version 1) works:
>>> c = SimpleCookie()
>>> c.load('a=b; c="c\\"c"; d=d')
>>> c.output()
'Set-Cookie: a=b\r\nSet-Cookie: c="c\\"c"\r\nSet-Cookie: d=d'
>>>

If you want to silently skip over invalid cookies by looking for a semicolon, see Issue 25228. For that, I think we need someone to add some more tests and confirm it doesn’t open a security hole.

Also maybe see Issue 22983 and Issue 17340.

Please find what Django's devs have done to parse cookies: 

https://github.com/django/django/commit/93a135d111c2569d88d65a3f4ad9e6d9ad291452

I hope this might help to find a solution.