Finalized OE19 events reverted to a prepared status for some employees

Our most recent open enrollment was passive -- employees did not have to take action to continue their coverage into the upcoming plan year.

We recently learned that finalized OE19 events reverted to a prepared status for some employees (500+). This is across various pay groups and benefit programs. Most employees have been employed on an ongoing basis.

Although we  have taken manual intervention, does anyone have any thoughts as to what may have happened and how to prevent this from happening in the future?  We are on 9.2, PT 8.55.