The introduction 'not' completely confuses the sentiment analysis and is really unusable in real life.
Example - "I didn't have a bad trip to Seattle. I enjoyed seeing the Space Needle!" returns 9% sentiment! Not really usable.
Assaf Israel [MSFT] commented
Thank you for the feedback!
We recently released a new sentiment model in preview which should improve on such cases. It also provides a breakdown of the sentiment by sentence.