That should be checked on the Access Gateway device where the NPIV dev is connected to because the switch provides very little info about that.
You may be able to check what you need by executing "portlogdumpport
You could grep the director's portlogdump for the fdiscs like:
portlogdump | grep fffffe
I just wonder if they would also explicitely log out by sending a logo to fffffe.
But: depending on how much is going on in your fabric, it could be that your portlogdump covers only a few minutes...