I have the same issue here. It seems to be due to the way the data are cached. Because, if I click on a novel which have the wrong number of chapters unread, it refreshes the data and shows the correct number of chapters unread. Moreover it happens only on books that I've not read in a while, so I speculate it might be the way webnovel app is storing data.
It might be due to an incompatibility on data architecture between two major versions or the way unread chapters are computed.
Also, another behavior I find distrubing:
It would also be helpful not to automatically reset chapters to 1 when someone mistakenly click on the first chapter of a novel. Maybe showing a prompt dialog or something to ask if the user really want that (or adding a reset button somewhere). Because it's annoying to find again the last chapter you read when it happens.