As touched upon earlier, it's not entirely unheard of, particularly where factors such as actor scheduling dictate things. Something similar happened with Revolution of the Daleks, which aired New Year's Day 2021 but was filmed October 2019 in order to accommodate Bradley Walsh's and Tosin Cole's schedules, particularly so they could leave and move onto other things.
And as it turned out, given all that stuff that went on in 2020, it was quite a stroke of luck they already had Revolution of the Daleks already in the can.