Please forgive me if I have this all wrong.
In Developer Tools / Template …
{{ state_attr('sensor.forecast_tomorrow','detailedForecast') }}
results in a list of dictionaries …
{'period_start': datetime.datetime(2023, 7, 18, 0, 0, tzinfo=datetime.timezone(datetime.timedelta(seconds=3600), 'BST')), 'pv_estimate': 0.0, 'pv_estimate10': 0.0, 'pv_estimate90': 0.0}
JSON Path Finder tells me there are JSON errors.
Could the value for the ‘period_start’ key be a simple datetime string, please?