Page index and chapter slip after rotation lock in viewer

[Peter]

I have some chapters of a manga downloaded, after reading ch1 I press < button and open ch2, then because I want to hold my tablet at 45 degree angle while reading in my bed, I click the rotation lock button and everything is OK before I try to flip next page, that is when I find my MangaDLR 6.1.2 viewer showing previous chapter that I have read already. Trying to replay the situation again leads to blank activity after returning back to manga folder and trying to open the finished chapter, next/previous commands do not work, only < key, I suspect it got page index out of range for the chapter.