Also tried toggling "AUTO" on. Not even sure what this means. Surely with a custom fan curve we are not operating automatically but manually?

Sounds like a piece of software is causing the fan to kick on/off when inside windows since you don't have the problem while idling inside the bios.
 
Do #1 listed here to see if it helps get rid of the issue.

I have had similar problems.
Do you always have the XOC running in the background? I've found that if i don't, my GPU fan will never turn on. You can simply press the downsize button and it will start running in the background. 
If this isn't the answer to your problem, please tell us.
 
~Regards, Rocketman.

Been a while, but NVIDIA nTune Command and NVIDIA Performance Service is part of NVIDIA System Tools..Did you install that trash?..Look in Control Panel -> Programs and Features and see if NVIDIA Performance and NVIDIA System Monitor is there..Get rid of them if they are.

NVIDIA System Tools/nTune has a long history of causing problems and conflicting with other software, so yeah, need to lose that so it's out of the way, it's bad juju..One more thing it installs though that I forgot about (until I installed it here) is 'NVIDIA Drivers' (not NVIDIA Graphics Driver) you need to uninstall too.
 
After you get rid of everything NVIDIA System Tools installed, I'd run DDU in Safe Mode to uninstall the NVIDIA Graphics Driver..Then install the NVIDIA Graphics Driver again..Probly should uninstall Precision XOC and delete the leftover files/folder in the Program Files (x86) folder and reboot and install it again too..Start clean..Roll from there.

That's cool, and you're welcome.

In case you don't know, something else that might help is that when you open the Fan Curve Editor in Precision (or Afterburner) you can double-click in a blank area of the editor to change the curve to steppings (and vice-versa)..You can remove setpoints by clicking to highlight them, then press the keyboard Del key..You can add setpoints by clicking on the line..Highlight and drag the setpoints where you want them or use the keyboard left/right/up/down arrows to move them more precisely..You already know about Temperature Hysteresis and the Fan Speed Update Period.

A lot of fan setpoints close together (temperature-wise) can cause unwanted fan speed fluctuations..I use only three setpoints, myself.

You can use MSI Afterburner if you prefer, it works pretty much identical to Precision..Your card features dB Inverter which turns the fan off when GPU temperature is below 60C, unless it's overridden by fan control software such as Precision or Afterburner when Automatic Fan Control is enabled in the Fan tab of the Settings (gear icon, bottom-right)..More here on fan control..It's also normal for the fan to twitch slightly when off, should you notice that, as stated here.

The 'AUTO' button on the main Precision screen toggles the fan speed from automatic to manual control and back again..When it's not lit (manual mode), the fan will remain at a fixed speed and will cause throttling/overheating of the card if it's fixed to low.
 
Whew!..OK..That's all..Good luck.

I would uninstall Precision, delete the leftovers which contain the old .cfg files, then reinstall it after rebooting, yes..Then set up your fan curve.
 
It would help if you made a signature that lists your hardware and OS..That way people don't have to search through your posts to find out..Go to 'User Control Panel' (top-right of this page), then 'Edit Profile -> Signature and Comment'..Whatever you put there will be stickied at the bottom of all your posts.