Resting Energy suddenly drops by 30%

I’ve been wearing an Apple Watch every day for 5 years. In that time, my resting energy has stayed in the range of 1850-2050 kcal, depending on my fitness level.

Yesterday, it suddenly dropped to 1400kcal, same thing today.

I have checked my health app body measurements, I have checked what devices can write data, I’ve restarted both watch and iPhone, no change. I also read the various articles on increasing my watch accuracy, nothing jumps out there either. As far as I know there hasn’t been an iOS or watchOS update that could have caused it, and no new updates are pending.

A search on the community shows this is not a new issue - people have had this problem in the past, however I cannot see that anyone has gotten a fix for it?

I monitor this data closely on a daily basis and I’m finding this unsettling - please help?