[South]

This seems to be a new thing now under the new Doctor Who, they film so far ahead of transmission. I remember during Tennant's first tenure, they would film a series over nine months, but it was always far closer to transmission than what they have now.

So, if they do continue to film so far ahead, which is what will happen as Ncuti is filming his second Christmas special already, it will lead to leaks happening.

Post production now takes alot longer due to special effects. Most sci-fi series now spend 9-12 months in post.
Series 2 filming started back in Novemeber so the Christmas special has already been filmed.