Personally, and this is just me, I suspect the powder. Sorry but I do. While range time with the powder is fun, something about all the quirks of this powder just rub me wrong. One reason when my supply runs out, that will be fine.

Both of the rifles you mention, unless you were rolling in the snow making snow angels (but at -6º that's not recommended), or standing in a rain storm are very water resistant rifle models. A tight sabot should stop what little moisture might make it down the barrel And the closed breech design of the gun, should have kept that area good.

When I had problems getting BlackHorn to ignite in my Optima, I got the same gambit of excuses, wrong breech plug, didn't scrape the carbon out with a drill bit, too loose a sabot, weak primers, and by the way your STS Remington is a HOT primers...

Now the cold temperatures should not be a culprit. After all if a rifle can not fire in the cold then to me its of no value. Condensation in the powder area, although the rifle was loaded fresh after the shot you say... so this is confusing as normally there is a clear fire channel, and the powder is dry. If you shot a rifle and then reloaded it. Unless you used a soaking wet patch and swabbed, which, with BH209 you don't have to do.

I personally suspect the powder. And I can't put a finger on why. I would hunt cold weather when ever possible with Black Powder #1 tried and true, Pyrodex RS, or Triple Seven. In that order. But I know the BH209 readers will not like my answer. I would test my theory, after all the wind chill here was -40º so that should qualify.. but I don't want to shoot in that kind of cold anymore.

BH209 resulting in fizzle = loose sabot (which it sounds like, yours is not.) Condensation in the powder... which you fire the rifle before this and it shot fine, then reloaded with dry powder and same sabot and got a fizzle. So that don't seem to be it. You're using a hot primer and have the right breech plug... so what's left? I suspect the powder failed with the combination you have. So I would change powders. I'd rather clean a rifle five minutes more, the miss a deer.