dropper post not working at freezing temperatures

I am speaking only from a third person point of view as I primarily ride on the road, but could it be the gas/fluid actuating your dropper is affected by the cold as that seems the most logical.

These two links perhaps may help and maybe your LBS or Rockshox warranty department can shed some light on if there is a known issue with your particular model.

A full hydraulic dropper post uses a sealed hydraulic remote to control the seatpost. The system itself works much in the same way as a hydraulic brake cable, whereby a piston (the lever) pushes fluid through a hose which actuates the dropper at the other end.

^{from https://www.bikeexchange.com/blog/buyers-guide-to-mountain-bike-dropper-seatposts}

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• It happens in the cold, whether bike is ridden or parked or on a rack, so its not just riding in the cold.
• Release all the air pressure then bleeding the hydraulics may help,
• Increasing/decreasing the air pressure may help, with a shock pump, figures of 250~260 PSI were quoted.
• Sideways play in the top or sag when fully extended may mean a rebuild with new seals, or a complete replacement.

^{From https://www.mtbr.com/threads/rockshox-reverb-cold-weather-issues.993072/ summarised.}

The reverb, much like Shimano brakes, has a known issue when operating in the cold. The oil inside simply thickens too much to properly flow through the lever and hose. The oil flow through the damper also gets constricted. I don't think there's really a solution because switching to a thinner oil might cause issues in the summertime, such as the post rebounding too quickly and damaging internal components. Of course, you could switch to a different dropper with a more modern design that does not suffer from that issue.

Do NOT pull up on the reverb when it is compressed, even if the lever is being pushed. You will suck air past the seals and cause the infamous "Reverb squish" (where the post is no longer capable of staying rigid thanks to air being mixed in with the oil, and you basically have a very expensive suspension seatpost. Cannot be fixed with Viagra.)

Edit: If your particular model has it, try turning the speed adjuster to full open/fastest. That might help.