Are you losing great-grandma at the very end of her life? Is it possible she married very late in life and her last name changed?
Took me a long time to find an ancestor of my wife who married her second husband in her late 60s. I assumed (incorrectly) that when her first husband died when she was in her early 60s that she never remarried. She didn't disappear, but instead died and was buried under her second husband's last name.