Chrono testing of loads with the powder positioned at the bullet base, lying across the case bottom and positioned to the rear (at primer) show the greatest velo spread and lowest velo with the powder forward (at bullet). That, combined with the low temps, probably is the culprit.
I have observed this by shooting identical loads, back to back, one tipped forward and fired, the other tipped back, and can hear the difference and see the change in point of impact through the scope.
The best compromise may be to keep the rifle horizontal, thus the powder is 'rolled' across the bottom of the case in a relatively uniform way, and if you raise it mostly horizontally to shoulder it may pretty much stay there. If bench resting you can chamber the round and roll the rifle side to side to distribute the powder within. Just be aware of this, and do it the same each time and you will have consistency.