Socialisation is often discussed as something you finish in puppyhood, and for a naturally outgoing breed that is roughly true. For a reserved one it is maintenance. The dog is not learning that strangers are wonderful; it is learning that strangers are unremarkable, and that lesson decays without repetition. Five months of quiet streets is a long time for it to decay in.
What owners here typically report is a dog that was settled about passers-by in October and is barking at them again in March. That is not regression in the dog's character, it is simply the absence of practice, and it is entirely predictable once you know to expect it. The mistake is treating the March barking as a new behavioural problem and paying to have it addressed, when it is a seasonal dip that recurs annually.
The fix is deliberate winter exposure at a cost of nothing. Walk the skyway-adjacent streets at the hours people are actually outside, around lunch and at the end of the working day, rather than at the empty hours the cold pushes you towards. Use the entrances to grocery shops, transit stops and the farmers market buildings as places where people flow past. Keep the encounters brief and non-interactive; the goal is exposure, not greeting. Ten minutes of that three times a week through winter is what preserves ten months of work.