From telemetry, my best guess is a perfect storm of tyre warm up issues for Leclerc along with his typical setup and driving style that stresses tyres less than Sainz, so I guess this also means less heating up.
So the track was getting colder and colder with no sunlight to warm it up as the session passed. Leclerc's Q3 lap matches T1-snail performance of Q1 lap, while Q2 lap was his best there today (not shown on graph). Add the long back straight and only two corners before the next straight and your tyres inevitably get cooled down and in my view Leclerc's setup made those tyres fall out the bottom of the temp window before he hit T1 in Q3. T1-snail in Q3 lost him 0.37s to Max and further 0.25s on the whole rest of the lap. If the track was as hot as Q1 in Q3, the gap would have been around 0.4s in my view.
Sainz on the other hand never lost so much time in T1-snail. In fact, his Q2 lap (like Leclerc and also not shown on graph) was his personal best there today and completely matched Max' Q3 time. However, he lost quite a lot more time on the rest of the lap than Leclerc every time, leading me to believe he lost less time today on colder track during Q3 than Leclerc. One difference was also quite evident in the Sprint today, he takes a different line to Leclerc there and this seems to work better in Q for a Ferrari, while making basically no difference in the race trim.
Since tomorrow the conditions will be the same as today (maybe even more cloudy), I expect the cars in front will struggle a lot less with deg as the race goes on, unlike Suzuka. On the other hand, Leclerc might suffer in the early laps with his slow-to-heat-up setup. Ferrari have little choice, they will both have to back off in the first stint, extend it forever and watch how the race unfolds and go from there. One stop might be both doable and a very good choice in their case. I think they would do well to go M-M-S as I said today, making M-M stints until lap 45 could be doable and then go for a trail blaze run on Softs for 10 laps.
I'm not expecting a podium due to cold weather, while Alonso might just pull a rabbit out his hat tomorrow and hold 2 McLarens and 2 Ferraris the whole race.