I agree with WH I've noticed mine do the same thing...in my experience I linked my drooping leaves before "sunset" to a nute problem

I run an aero9 and I'm at 6 weeks flower 1050ppm at 5.2ph I tried to go higher but the plants just aint havin it with sucha low ph...but growth is looking very healthy so I'm sticking with it...I noticed mine get really dark green and in the cola leaves were curling and they wouldn't reach as much for the light if ppm was higher...

I'd say if your leaves look and feel healthy including the top of the cola yer probably just maxing out the nute intake so it's not the end of the world don't worry unless the leaves stop coming up ;-)

I noticed recently in the last 2 weeks of flower that the ph is dropping faster over night and being relatively stable during the day so I've dropped in 3 hours into the dark to ph up to 5.2-5.3 and 3 hours before light to ph up again to 5.2-5.3