At this thread (Awaiting Biometrics (Stage 3)) you may want to read posts from early 2023 onwards, to have the answer to your question.
Basically, in Dec 2022, preapprovals from applications in November were invited to choose dates within January to March 2023, and thereafter early December applications were invited for April/May 2023. In all probability, towards the end of this month, the next “batch” will be invited for June/July.
Whether the next batch is the remaining Dec 21, or part thereof, is a guess. Dec 21 seems to have such has a large load of applications, that the last applications do not have pre-approval yet.
When you’re invited, choose a date that you do not miss, and by which you can get all your documents updated, if possible.