If an item has the setting "Lock Item after failure" at the item level "Failure Action" settings with a maximum number of attempts mentioned at "Times user can fail before action", after that attempt the item would be locked in the user's learning plan.
If the item is removed form the Learning Plan and reassigned, it would not have the Object Details and would be "Locked".
This is a different item lock message(KBA - 2316896, not in the Learning Plan but in the Content Structure page) and to get this type of issues fixed, please increase the number of attempts in the Content object Settings by one value(changing this will not reassign the items to the users and it is okay), ask the affected users to launch the course(this will give them one attempt) which would enable the object details and once that is done, change the number of attempts back to the way it was.
This is not an issue, but any item should not be removed from the user's Learning plan directly(especially when it has the Lock/Remove action) as the association would still be there in the DB and forcing it to be removed and re-assigned would cause issues like this as there are data disassociation leaving some history behind and then a new assignment is performed which the system would try to associate if from where it had left behind and luckily we have workarounds for all of the scenarios(this KBA and 2316896) that could raise because of this.
Item Locked, course locked, locked, LMS, Learning, Learning Administration, This item has been locked , KBA , LOD-SF-LMS , SuccessFactors Learning , Problem