Did you fill up the bottle 100% and put a lid on? It could be water vapor in the air condensing.
which it is guaranteed to do below 0C (exactly)
Firstly it’s not exactly 0°C, but depends (very slightly) on pressure, secondly that applies only to pure water.
Water in a solution can have a significantly lower freezing point if an antifreeze agent is added. Since popular antifreeze agents such as ethylene glycol are carbohydrates relatively similar to gasoline, I’d expect a solution of a little bit of water with fuel to freeze significantly below 0°C.
Probably not low enough in the context of IFR flight which can easily see -20C, however.
BTW I didn’t think the freezing point was very pressure dependent. The boiling point is massively so, however. I vaguely recall the “steam tables” from college 
The BA problem was caused by a filter.
Was it?
Probably not low enough in the context of IFR flight which can easily see -20C, however.
According to this a mixture of 60% ethylene glycol and 40% water freezes at -45°C, which is interestingly less than pure water or pure ethylene glycol.
According to this, gasoline dissolves about 0.1% water. I cannot find any treatment of gasoline freezing characteristics versus dissolved water, so I cannot imagine this being a problem.
However, there are a lot of articles about water in gasoline-ethanol mixes, where water creates a solution with ethanol and this solution separates from the gasoline.
So if I were you I’d be more concerned about water in your Scirocco’s tank.
I have had many engine failures due to fuel icing. I can pretty much guarantee that if I fly for an hour at < -20°C my right engine will stop.
However, if I use IPA (similar to Prist, but less corrosive and dangerous) I never get fuel icing.
I now use 1% IPA at all times. It is the same price as Avgas anyway, so it doesn’t matter, except for minor inconvenience.
That, however, should absolutely not happen with clean avgas, whose freezing point is way below that.
So why does it happen to you?
You either have accumulating water somewhere, or your avgas has water in it.
An Aztec cannot possibly be airworthy if an engine stops below -20C. There has to be something severely wrong with your fuel system.
I have flown for hours at -35C or so, without any issues. That’s without the electric fuel pump or alternate air. But it was nearly always in VMC.
And if you do get water in your sirocco’s fuel system the trick is to run ethanol through it to get the water out.
