Reddish brown pistils, end of week 3 of flower.

Have you been foliar feeding during flower? That can turn pisitils brown

One of mine is doing that too. Is it a fast flowering strain? Like 54 days? Sativa doesn’t mean much man. It all depends on how long it takes. I’ve seen some sativas finished before Indicas So does that make the sativa an indica now? That feeling of up and or down you get when your high has to do more when it was cut down rather than the strain

I exchange air direct from a vented loft space so humidity isn't an issue. Rarely peaks above 45% thankfully. The appearance of the pistils is no reason to worry? They seem off?

I wont be concerned at all, but it looks very crowdy in your tent already can you keep the humidity low=? because of so much transpirating Plantmaterial

It's not on just one plant. It seems prominent on the upper canopy but can be seem in lower amounts below.