Athosanswered grow question 7y ago No, it's not normal, but hardly harmful, if anything it's good to have low humidity in flower, less chance of mildew or budrot.
Plant transpires almost 100% the water they drink, that means that as they explode with growth during the pre-flower stretch and they drink more and more water, they will release it into the environment making RH rise.
That been said, humidity which is actually relative humidity changes a lot with temperature, 60% humidity at a certain temperature can drop drastically if the temperatures increases just a little. Perhaps your weather has changed and the air is drier than a few days / weeks ago?