This isn’t surprising. When a relative dies you have a lot of things to deal with. The last thing you would think to do is to inform the voter registration board of the death. In fact, that might not be a very smart thing to do as who knows what they would do with that information.
Better idea if you can is to go to the polling place of a dead relative and make sure no one has voted in their name.
When a notice of death is issued a copy should be sent to
the voter registration keeper of in jurisdiction.