Heating Activity report misbehaving

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Is there something up with the Heating Activity data today?

I've been keeping an eye on it over the last few weeks and it's been pretty consistent, although how accurate it really is I can't be sure.

However today it's been all over the place.

My heating had been on for 2 hours this morning, and yet the heating activity said 0 hours. I checked it mid morning and it had shot up to 7 hours, by midday it had changed (down!) to 5.5 hours, before jumping to nearly 11 hours this afternoon. Just checked again now and it's dropped back to 5.5 hours again.

I've not seen it misbehave like this before and assume it's a bug of some kind? Is this calculated on the server side or locally on the devices and then accumulated?