When deleting a slackloop, the cable length fields (including CableLength, GlassLength, SegmentLength and SegmentGlassLength) are not updated appropriately.
This is likely related to a limitation in Fiber Manager functionality. When a slackloop is deleted, Fiber Manager does not know which cable it was attached to and lengths for the corresponding cable are not updated.
This problem is resolved in ArcFM 10.1.
Ensure the 'ArcFM Fiber Optic Cable Length' autoupdater is assigned to the 'On Feature Update' event for the SlackLoop feature class. Before deleting the SlackLoop, update its length and set it to 0.