From Dec 18, the way in which ADS-B aircraft are counted by the stats system was changed.
The aircraft counts before the 18th are not comparable to the aircraft counts after the 18th.
If you see a change in your ADS-B aircraft counts starting from the 18th, this is normal. The data that you’re feeding to FlightAware has not changed and is still being used in the same way. This is only a change to the stats system, to produce counts that are consistent with multilateration aircraft counts, and to fix the “total” value which was not reliably counting aircraft.
Stats shown for the 19th-21st are, at the time of writing, a mix of the old system and the new system. They are being reprocessed now under the new system. This should be complete within about the next 24 hours. Once that is done, everything since the 18th should be on the new system.