Edited: there is a radio button to mark the CLN as “certain” or “uncertain “ (my apologies)
Although it is my practice to populate the cln field with the LNAB, unless you have sighted the death certificate, you cannot be sure of the cln. The person may have changed their last name for any one of a plethora of reasons- adoption, marriage, remarriage, divorce, immigration, deed poll etc
Where I live, there are many women who retain their LNAB after marriage. Sometimes, the man changes his name. Sometimes they both change (eg use a hyphenated version)! It is also legally possible to revert to a former name, or to continue using the name of a former partner . Etc etc