A record changes from one segment to another but no Change Segment activity is found in their Activity Log.
When a segmentation is updated, either adding or removing filters within a segments, and then the draft is re-approved, Marketo does not log a Change Segment activity for records which now belong to a different Segment under the new rules. This is because the record technically didn't change their segment, but it now belongs to the appropriate segment based on the filters using the newly approved configuration. In other words, the record didn't change, the segmentation changed.
If your team is utilizing trigger campaigns to qualify records based on a segment changing, it is advised to create a batch campaign after the new segmentation configuration has been approved to update those records that would have qualified for those triggering campaigns.