Both edit3DENMissionAttributes & set3DENMissionAttributes will report unknown enum when using custom attributes.
Snippet for edit3DENMissionAttributes
18:15:01 Error in expression <edit3DENMissionAttributes 'TMF_MissionBr> 18:15:01 Error position: <edit3DENMissionAttributes 'TMF_MissionBr> 18:15:01 Error Foreign error: Unknown enum value: "TMF_MissionBriefingAttributes"
Snippet for set3DENMissionAttributes
18:15:07 Error in expression < _array; private _string = str _array; set3DENMissionAttributes [["TMF_MissionB> 18:15:07 Error position: <set3DENMissionAttributes [["TMF_MissionB> 18:15:07 Error Foreign error: Unknown enum value: "TMF_MissionBriefingAttributes"