Length of Service formula does not calculate correctly

Summary:

We have issues with the formula to calculate incorrectly the length of service.

Content (required):

The following formula is calculating the Length of services however it does not return correct results on all records and we need to validate each record. I attach the screenshot of some incorrectly displayed results (First column is Enterprise Hire date and the 4th column is the calculated Length of service. I blurred the results that were shown correctly and opened the incorrect ones ). Any idea why this formula does not work properly and how we can fix it to have the constant and reliable results.