Possible answer:
Any update on this issue? I have encountered the same thing. TandemSeven user has almost 8000 tweets since April 2009. I use screen_name='TandemSeven', count=200, exclude_replies=False, include_rts=True options and as far as I can get is to 1592 tweets.
The most recent tweet created at Thu Jul 26 12:18:06 +0000 2018
The latest obtained tweet created at Fri Jul 22 03:00:06 +0000 2016
Is it possible that there have been over 1500 tweets deleted from the timeline?