This appears to be an individual case/bug, since as soon as a book is marked as 'currently reading' it's removed from your TBR regardless of whether it's marked as Up Next or not. Did you start reading the exact same edition? I also wonder whether for the book in questing there might be a duplicate read status in the backend somehow. It's best if you email in to support for this one, as it's easier for us to me to take a look at your records and have a back and forth if needs be.