FWIW I have heard that this spike appears to coincide with Christmas, and allegedly this has happened in the past. It’s possible that this is due to users on the platform receiving robux or premium for Christmas which is having a knock-on effect on the Robux distributed for premium payouts.
The purpose of making this bug report was to make sure that this is correct (I’m now more sure that it is correct), and to provide a single place to get an official statement on this so this hopefully isn’t reported every single year (since a few threads about this were starting to appear yesterday).
I’m going to mark this as the solution in the meantime.
I checked with our economy teams and can confirm the reason for this spike is that we see a bump in Premium subscriptions during Christmas, so there is an expected increase in Premium payouts ~a month later due to how the engagement-based payouts are calculated.