Activity recording - outdoor cycle.

Something has happened to my outdoor cycle activity recording. A month ago, an 80 minute ride would tell me I had burnt roughly 700 active calories. This has been the case for the last couple of years. All of a sudden the same ride is giving me a figure of roughly 400. Distance/speed/heart rate are broadly the same. Other activities - walking, indoor cycle are giving me the read out I would expect. Spent an hour on the phone to Apple who couldn’t help. Any thoughts/solutions? Thanks