Wait im not sure what you mean, its 40%rh when lights on then it goes to 65-70 at night?
I was also worried about high rh, those buds, I hope im wrong but they just have that infected look. Id take a close look inside a brown cola and take a smell and smell for mold.
My rh never passes 60 and its packed, I used to have that swing at lights out too I have no clue why it stopped. All I know that time I severely overpacked a 2x2 and got mold.
Thought that was mostly why the rh was swinging being too packed but yours is not packed so idk whats making it swing. 24/0 lighting should help stabilize it though.