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.