I'm running into the same problem (deleted my mobile boarding passes including an LGB-SJC one), and my flights are about 5 months old. It seems like B6 never credits to SQ without you submitting a mileage claim.

I looked at flights through ITA Matrix, and the cheapest LGB-SJC flight ($44) seems to book into P. From pricing out current flights ($ rounded up), it seems like $44 = P, $49 = S, $59 = U, $69 = O, $75-79 = Z. Those all correspond to 50% earn. M is where it starts crediting 100%, and I see that $86-89 flights are falling into that fare class ($98 = W, $110 = R). Not sure if that'll be helpful, since you were on a presumably more expensive Christmas Eve flight.

Anyways, I'm about to submit my claim with my approximate guesses at my fare class based on what I see on ITA. I'm hoping they won't care if I get the class wrong, but it still falls into the same earn % (all my flights should fall into only 50% earn).