HUGE daily calories drop on Apple Watch

Ive had my Apple Watch (Series 6) for about 3 years now. Im a fairly active person, I go to the gym 4-5 times a week, I walk a lot, I have a physical job, and up until 19th November I was burning over 1,000 calories a day. Since 19 November, according to my watch, I have struggled to reach 500 calories a day despite not changing my routines.

Is this a software issue, an update issue, or a widespread issue? Anyone else had the same problem?