Gives me an error on the configuration check:
Configuration warnings
Invalid config for ‘history_stats’ from integration ‘sensor’ at configuration.yaml, line 35: invalid template (TemplateSyntaxError: unexpected ‘)’) for dictionary value ‘start’, got ‘{{ now()) - timedelta(days = 1) }}’
I had a lengthy discussion with the code owner a while back about history stats. One of the takeaways from that was that if there is absolutely no history before the start of the timeframe, the results are wrong. In that case the assumption is made that the first state it sees in the timeframe also applies since the start of the timeframe (before the sensor even existed), explaining the 23 hours on. So tomorrow the sensor should behave better.
And as Tom said, because you’re using a timeframe of the last 24 hours instead of today, the value can decrease. So during today it will decrease until it is correct.