Solution : https://service.sap.com/sap/support/notes/307189 (SAP Service marketplace login required)
Summary : 
      SAP Note highlights issues during upgrades impacting the MV45AFZB user exit, advising on its reset to initial state without user-defined code when moving from Release 3.x to any Release >= 4.0. Specifically, the problem pertains to upgrades using Release 4.0 as a technical step. For Release 4.7 or higher, regardless of initial release, users must adjust using Transaction SPAU due to elementary changes in the upgrade process, affecting several user exits such as MV50SFZ4 and MV45AIZZ. See the upgrade guide for details.
Key words : 
shipping shipping sales & billing document mv50sfz4 mv50afz1 mv45afzb lv50r_viewg02 mv50afz2 mv45afzz lv50r_viewg03 mv50afz3 mv45aizz lv50r_viewg06 mv50afzk mv45aozz lv50r_viewg07 mv50afzl mv45atzz v50rztop mv50afzz mv61afza lv50r_viewg04 lv50pfza rv60afzc lv50r_viewg01 fv50uzxx rv60afzz lv50r_viewg10 lv53sfz1 rv60fus4 lv50r_viewh19 lv53stz1 rv45pfza lv50r_viewg05 lv50r_viewg14 lv50r_viewg08 lv50r_viewg09 lv50r_viewf63 lv50r_viewf62, mv45afzb user exit, basis component bd-upg, user-defined source code, made user-defined modifications, mv45afzb object, user exit reason, transaction spau due, user exits, longer exists
Related Notes : 
       
| 637683 | Add info about upgrade to SAP R/3 Enterprise Core 4.70 Ext 2 | 
| 607893 | User exits in transaction SPAU | 
| 587896 | Add. info on upgrade to SAP R/3 Enterprise Core 4.70 SR1 | 
| 513536 | Add. information on upgrade to SAP R/3 Enterprise Core 4.70 | 
| 381348 | Using user exit, customer exit, VOFM in SD |