after the last but one update I noticed CF changed the way it makes segmentation. Despite the segmentation rule is set to "sentence" it often fails to begin a new segment with the following sentence.
Is it an intentional change to this or just a bug?
Nothing has been changed regarding the segmentation in the 2017 updates.
If your project was not created/segmented in another tool, the Sentence segmentation should work fine. The segmentation process takes place when you create a new project. You cannot change it (re-segment) afterwards.