Uncomfortable Idea: When people are telling you all about your loved one who has passed away, you are almost certainly hearing a highly fictionalized or at least heavily biased version of reality.
There are several cognitive errors here, but choosing to only say nice things about the dead to those who were close to the dead, is not one of the errors. Remembering grandad as a wonderful man full of ideals and integrity might make you feel comfortable, and that's great. But if truth and reality are important to you, you might want to acknowledge these biases and know grandad for the person he actually was, faults and all.