A few indexes became unusable after data purge

Hello for whom watching this case

Recently, I suffered an issue after upgrade DB from 10g to 12c that a few indexes of partition tables will became unusable after I perform data purge on these tables. The interesting thing is that these indexes are not always became unusable after data purge but randomly.

Now, I have to check and rebuild these unusable indexes once perform data purge but I'd like to know the root cause and to see if anyway can fix it except rebuild.