First, that ain't normal. That's not even approaching normal.
For years, I've heard people talking about how their guitar problems couldn't possibly be due to humidity, since they're vigilant about keeping it at that magical 45-50% level. Well, that's all well and good, but too many people simply think they can maintain that humidity and everything will be fine. They don't check their guitars from time to time.
Second, if I were to make a wager, I would bet that the fretboard has dried out. Yes, I read about the lemon oil and the humidity levels. The fretboard dried out despite those. Those cracks didn't occur overnight. Those cracks happened over time...