Quote from Alimeru
So it turns out that if you have a manga set to 5 chapters read, then use the -5 button, the number of chapters read will be set to 0. This isn't possible if you use the -5 button on a manga that has been set to =/=5 chapters read. You also cannot use the Edit button to manually set this number to 0.
I personally don't mind this "bug", and unless it's causing any backend problems, I think it should be up for permanent implementation as others have suggested in the past that it would be nice to start the chapter count at 0 since some groups like to label releases (such as prologues) like that.
I just realized I commented this on the wrong thread (the API bug report one). You can delete my comment in the other thread since it's not relevant.