It sounds like the same thing that is happening to a lot of people. Someone from your tree that use to be on your thru lines page is no longer there, but instead is just the blank figure of a man or woman marked "Private." If that is the case, it was probably caused by someone recently taking a DNA test and linking it to a private tree that contains the disappearing person. All it takes is one person with a private tree to do this. I have brought it to the attention of Ancestry and they said they would look into it. If this is not fixed, the usefulness of thru lines will erode over time as more people link private trees. If people want their trees to be private, fine but that shouldn't spoil the usefulness of thru line for the rest of us.